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.
db2pd - a great little tool for inspecting a running DB2 instance for everything from table usage to lock status. This is the best DB2 tool I've found since DB2 Monitor. (Probably the best one I've found ever.)
Props to Craig Eaton for educating me (and many others) about it here.