From: Overview of biological database mapping services for interoperation between different 'omics' datasets
Mapping services
Link
Gene/Clone ID converter
http://idconverter.bioinfo.cnio.es/
ID mapping by UniProt
http://www.uniprot.org/?tab=mapping
MatchMiner
http://discover.nci.nih.gov/matchminer/MatchMinerLookup.jsp
DAVID gene ID conversion tool
http://david.abcc.ncifcrf.gov/conversion.jsp
g:Convert
http://biit.cs.ut.ee/gprofiler/gconvert.cgi
CRONOS
http://mips.helmholtz-muenchen.de/genre/proj/cronos/index.html
bioDBnet:db2db
http://biodbnet.abcc.ncifcrf.gov/db/db2db.php