Open Source OLAP Solutions?

webmaven asks: "After reading a thread on the local LUG mailing list on databases, I started wondering if there were any Open Source OLAP (On-Line Analytical Proccessing) database solutions out there. There are plenty of RDBMS OLTP (On-Line Transaction Proccessing) solutions, including MySQL, PostGreSQL, Interbase/Firebird, etc., and also plenty of Open Source Object database systems, but I can't seem to locate any Open Source OLAP databases. If there are any out there, I'd like to hear about them, and if there aren't any I'd enjoy any speculation as to why not."