Citebase Search was an experimental, semi-autonomous citation index for free, online research literature created at the University of Southampton as part of the Open Citation Project.
[1][2][3] It harvested 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.
[4] Citebase went live in 2005[1] and ceased operation in 2013.
[3][5] More than three-quarters of the papers indexed were author self-archived in the ArXiv archive, which includes physics, maths and computer science.
[6] Some (published) biomedical papers were indexed from BioMed Central and PubMed Central.