Which databases support parallel processing across multiple servers? I need a database engine that can utilize multiple servers for processing a single SQL query in parallel. So far I know that this is possible with the some engines, though none of them are feasible for me either because of pricing or missing features. The engines currently known to me are:

MS SQL (enterprise)
DB2 (enterprise)
Oracle (enterprise)
GridSQL
Greenplum

Which other engines have this feature? Do you have any experience with using this feature?
Edit: I have now proposed a method for creating one myself. Any input is welcome.
Edit2: I have found another one: Informix Extended Parallel Server
Edit3: Stado is a new one.
Edit4: Maybe also pgpool-II, they write:

Parallel Query
Using the parallel query function, data can be divided among the multiple servers, so that a query can be executed on all the servers concurrently to reduce the overall execution time.
Look into Hive:

http://wiki.apache.org/hadoop/Hive

It's free and scales very well.

A new presentation was posted recently with updated numbers:

http://www.slideshare.net/ragho/hive-icde-2010

also see

http://wiki.apache.org/hadoop/Hive/Presentations