adodb
Alex is porting Tasks to use ADOdb from straight MySQL. I've built HelpSpot from the ground up using it. I've been using it for several years and I'm really happy with it.
He mentions the trouble Postgres can be if your moving from a MySQL/SQL Server way of doing things. HelpSpot won't be supporting Postgres initially, mostly because of a lack of experience with the platform. Hopefully down the road we'll be able to add support for Postgres and Oracle.