For django, I've been using django-compress: http://code.google.com/p/django-compress/

Works with javascript as well as CSS, nicely incorporates cache control, ahead-of-time or on-the-fly generation, etc. Works out of the box* in pure python, or it's trivial to plug eg YUI minifier or google-closure.

* Actually, last I looked there was a bug in the CSS minifier it ships with (http://code.google.com/p/django-compress/issues/detail?id=27), but I believe the upstream version has been fixed.