Raw Therapee is excellent and rivals both ACR and Lightroom.
I make my living teaching photography and Photoshop. When I teach a photography class and the students can't all afford to buy Photoshop I have them use GIMP. In fact I insist that they can only use Photoshop or GIMP. The first big hurdle I face in a photo class is to stop my students from all the bad practices they've adopted using bad software.
When I finally retire and stop teaching I won't have to have Photoshop and if at that time I can't afford it I'll switch to GIMP and won't look back with any regret for dumping Photoshop. GIMP can handle what needs to be done. A little less elegant, but capable.
GIMP is an excellent program, and when you do the guzintas for cost vs. benefit, it blows Photoshop out of the water. It isn't quite as polished as CS5, but it will do almost everything you need to do.
I'm going to give you some argument on that one. Picasa is one of the products that I have to wrest from the hands of my students and get them to stop using. Picasa does not provide a color managed work environment. That's a critical flaw.