Greenplum (I think)
Of course, the old guard is well-represented in this category as well - Oracle, DB2, SQL Server, Teradata, etc. all allow language extension via custom functions, plug-ins, etc.
I don't know enough about it yet to know, but I think that Greenplum and AsterData might also belong on this list due to their support for Map/Reduce.
It wasn't all that long ago (3 or 4 years) that most of these vendors didn't even support the full SQL standard, never mind compiled-code extensions to their SQL language. Oh, the possibilities.