Update: for a little background on our specific interests - we wish to build a deployable and distributable version (a package or vm) with a preconfigured and loaded current ChEMBL database, capable of performing full chemical search capability. Deployment could be as a Linux style package, or as an Amazon EC2 instance. Our internal systems here at the EBI are based on Oracle, and the MDL (or whatever the current name is :) ) Direct cartridge - this configuration is sometimes beyond the reach of many budgets, and so we are interested in exploring a 'free' but useful version of ChEMBL.
Update 2: So Postgres opens up quite a few more options....
Comments
http://mychem.sourceforge.net/
MyChem is developed by Jerome Pansanel, and with OrChem (Mark + Christoph, EBI) part of the ChemiSQL project, that aim at creating a unified API independent of the actual database platform.
http://ggasoftware.com/opensource/bingo
(however, the license looks an issue here, maybe)
Both support substructure and similarity searching and are actively maintained.