Is there a good casual online place to practice/learn bridge?
If you are looking for free online site, I would strongly recommend BridgeBase Online, or BBO for short. 

You can create an account by just specifying a username and password and either login using the web browser or using their downloadable windows version of the software.

BBO has a lot of features, like partnership bidding rooms, GIB robots (which you have to pay a minimal fee, if you want to use them), tons of free tournaments. They also have two 'clubs', the Main Bridge Club and Relaxed Bridge Club, where you can play casual games (the Relaxed Club being more casual). Anytime I have logged on, there were at least 3000 players online! 

There is also the added bonus that a lot of great players play there (look for some of the 'stars') and they have an excellent broadcast line up of high level bridge tournaments, with commentators giving insights, etc.

They also have a supporting forum where a lot of good players visit and you can discuss bridge/the BBO software/ or even Global Warming!