Alternatives to Web of Science for cited reference searching were described in "The emergence of competitors to the Science Citation Index and the Web of Science", D.L. Roth, Current Science (2005), 89(9),1531-1536 [http://www.ias.ac.in/currsci/]. This article was included in a special section entitled: 50 Years of Citation Indexing.
Some recent relevant references include:
Three options for citation tracking: Google Scholar, Scopus and Web of Science
Nisa Bakkalbasi, Kathleen Bauer, Janis Glover and Lei Wang
Biomedical Digital Libraries 2006, 3:7 doi:10.1186/1742-5581-3-7
The following physical science databases as well as their options for citation searching were described: Chemical Abstracts/SciFinder/SciFinder Scholar, NASA Astrophysics Data System, Scopus, Scitation/Spin Web, PROLA (Physical Review Online Archive), Optics InfoBase, CiteSeer, IEEE Xplore, Spires HEP, IOP (Institute of Physics) and CrossRef.
Since the publication of this article, there have been some revisions and several new players.
All the content from the Instrumentation database has been merged into the Physics database
NASA ADS has added full author name searching (Surname, Firstname Middle Initial). Searching on the full name retrieves, for example, Smith, Charles A., Smith, C.A. and Smith, C.
At the bottom of the listing of an author's papers, following record/s selection, there are a variety of options (e.g. find similar papers, get reference lists, get citation lists, exclude self-citations, etc.)
What's New at ADS is regularly updated at:
Scopus has introduced a new 'Citation Tracker' feature that is available under the 'Author Search' tab. While designed to algorithmically differentiate authors with similar names and initials, there are multiple examples of authors (e.g. R.A. Marcus, A.H. Zewail) that have multiple registrations (even with identical institutional addresses). The initial author display also includes authors with reversed initials (e.g. H.B. Gray and B.H. Gray). A comparison search on citations to a Zewail article in Science 2004 retrieved 27 citations in WoS and 22 in Scopus (which didn't list articles in the March 2005 Phys. Today, June 2006 Rep. Prog. Phys., and May 18, June 22, June 29 issues of J. Phys. Chem. A). A search for J.C. Venter's paper on the 'Sequence of the Human Genome' displays his last name as 'Craig Venter').
Scitation has expanded its citation searching feature to include the CrossRef database. At the top of the Abstract page there are links to: Scitation Citing Articles, CrossRef Citing Articles and All Citing Articles. This feature is freely available to non-subscribers.
PROLA (Physical Review Online Archives)
PROLA refers to the archival file, which currently includes 1893-2002. PROLA has expanded the citation searching feature to include the CrossRef database. A 'Show Articles Citing This One' link is provided at the end of the abstract record. The bibliographic results are presented in reverse chronological order and the default gives both PROLA and CrossRef citations. Additional results are grouped by citing journal and year cited, which are ordered according to the number of citations, not alphabetically or chronologically. A 'Show Only APS Citations' link is also available. While full-text searching and displaying citations (including abstracts) is free, subscriptions are required for display of full text and citing articles.
The APS current file (currently 2003+) has a different display and offers links to: Citing APS & Scitation Articles, CrossRef Citing Articles and All Citing Articles.
Google Scholar has added a 'recent article' link (which limits retrieval to 2001+). A pull down 'year' menu is newly displayed showing 'since 2001', and can be changed to 'since (any year from 1987 to 2006)' or anytime.
CiteSeer document pages now link to ACM Portal [ http://portal.acm.org/dl.cfm ] records plus DBLP [ http://www.informatik.uni-trier.de/~ley/db/ ], a computer science bibliography containing over 770K records from conferences, journals, series & books.
ScienceDirect now provides access to 'Article in Press' with a link on each journal page above the issue list. These peer-reviewed manuscripts are either 'In Press, Corrected Proof' or 'In Press, Accepted Manuscript'. 'Corrected Proof' manuscripts are included in the results after clicking the 'Cited by' link.
In addition to conference papers, SPIRES HEP will also uniquely include citing papers from arXiv and Ph.D. theses.
Institute of Physics (IOP)
Individual articles in IOP journals provide a 'Citing Articles' link from the journal issue/volume contents page. A recent comparison search for articles citing:
R W Wood 1902 Proc. Phys. Soc. 18,269-275 give 7 citing articles. The same search in Web of Science gives 15 articles, 7 of which are in common, 4 recent articles from Opt. Express, Opt. Commun., Appl. Phys. Lett., IEEE J. Quantum Electronics; 1999 articles from J. Mod. Opt., & J. Lightwave Technol. & articles from 1991 and 1986.
New Players include:
Windows Live Academic
Windows Live Academic is a component of Windows Live, which also includes: Web, News, Images, Local, Feeds, & Products, and is currently limited to computer science, physics, electrical engineering, and related subject areas. Search results are default ranked by relevance. They can be reranked/limited by date-oldest, date-newest, author, journal & conference. The article title is hot linked to the publisher's website. Mousing over the citation (left colume) displays the article abstract in the right column.
Bibtex, RefWorks and EndNote formats are provided. Local SFX links are provided for cooperating institutions. Truncated citations (e.g. R.A. Marcus, Discuss. Faraday Soc. 29) can be searched to retrieve articles citing this reference.
Royal Society of Chemistry
The Royal Society of Chemistry has introduced a 'Forward Linking' option which provides 'Search for citing articles' links, at the article level, to citing articles in the CrossRef database. The CrossRef references include DOI links to the full text of the citing article. This feature is freely available to non-subscribers.
The RSC reference linking facilities also allow access to the Chemical Abstracts Service abstract for the reference via ChemPort and links to the full text of the reference, where available online, on the publisher’s server (via DOI identified using Cross-Ref).
Abstracts in MathSciNet include a 'Reference Citations' link to other MathSciNet articles.
Each PubMed citation has a 'Links' option which includes "Cited in PMC", if available, which links to citing articles in PubMedCentral. PubMed also has a 'Cited Articles' link to a PubMed list of the references in the original PMC article.
PMC is a searchable full text database that provides 'reference' and 'reference author' searching.
Reference - searches for words and numbers in the titles of journal references.
Reference Author - searches for author names in journal references. Since PMC references don't list full author names, use last name and initials (e.g., "fauci as" or "o'brien jc jr"). Initials and suffixes may be omitted. PMC automatically truncates author names (e.g., "o'brien j [au]" will retrieve o'brien ja, o'brien jb, o'brien jc jr, as well as o'brien j). To turn off this automatic truncation, enclose the author's name in double quotes and qualify with [au] in brackets, e.g., "o'brien j" [au] to retrieve just o'brien j.
The Abstract/Citation screen provides 'Most recent citing papers (via CrossRef)'
The Abstract display offers 'articles citing this article' in Google Scholar.
Citation Tracking allows researchers to track where an article has been cited in other online content, based on data from CrossRef. A license to view the full-text content of a journal article is required to view a list of links to citing articles. The feature will eventually be included for Online Books.
Access a journal article ABSTRACT and click on the Citation Tracking link in the article navigation bar. Citations for Wiley InterScience content are displayed separately from citations from other publishers.