Citebase Search is an experimental, semi-autonomous citation index for the free, online research literature created by Tim Brody at the University of Southampton, UK. It harvests open access e-prints (most author self-archived) from OAI-PMH compliant archives, parses and links their references and indexes the metadata in a Xapian-based search engine. Citebase went live in 2005.
More than three-quarters of the papers indexed are author self-archived in the ArXiv archive, which includes physics, maths and computer science. Some (published) biomedical papers are indexed from BioMed Central and PubMed Central.
A service displaying exclusively ArXiv internal citations is XStructure.
References
Citebase Wikipedia(Text) CC BY-SA