I know that Pogo.com (which powers Yahoo Games) has a long running online bridge game and that there are a handful of places for serious tournament/MasterPoint level bridge players online (though most seem based on software written a decade or more ago) but are there any places to play, practice and learn bridge in a more casual manner? Ideally via a well designed modern piece of software?
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!