I've been using Idiorm - http://github.com/j4mie/idiorm (used it on Weekis - http://weekis.com). It works really well for small-medium scale apps.
I can't be bothered with the bloat of some of the PHP Frameworks out there.
I like the look of DumbledORM, although it takes a bit of a different approach to Idiorm (and its "child" project, Paris - http://github.com/j4mie/paris - which is an Active Record implementation with support for associations built on top of Idiorm in ~130 lines of code). I'm not really a fan of code generation at the best of times, and for a tiny ORM it seems a bit out of place.
I do like the idea of being able to save multiple rows, jQuery-style. I might just steal that..