http://www.ensembl.org/biomart/martview

UniProt query = 18,547

http://www.uniprot.org/uniprot/?query=database%3A%28type%3Aensembl%29+AND+database%3A%28type%3Ageneid%29+AND+reviewed%3Ayes+AND+organism%3A%22Homo+sapiens+%28Human%29+[9606]%22+AND+database%3A%28type%3Akegg%29&sort=score

database:(type:ensembl) database:(type:geneid) database:(type:kegg) AND reviewed:yes AND organism:"Homo sapiens (Human) [9606]"

Biomart (linked to by Sean above) is pretty much your first port of call for all id-mapping problems.

In this case, ensembl also has a REST endpoint for gathering cross refs:

http://rest.ensembl.org/xrefs/id/ENST00000288602?content-type=application/json;