I released SharDB (Download) last week as a beta release intended for larger WordPress MU installs using an existing 1-3 character MD5 hash (by blog id) based multi-database sharding structure. (Other structures will be added in early 2010.) The alpha testing of SharDB was carried out this fall with the help of a few people who I knew were using the same multi-database that we were. SharDB has been powering live sites since early October.