I really dig Sandbox.com
It's $4.95/month but you can play every sport they offer (should you not want to just restrict yourself to baseball). there is a good community of people who are active and the coverage is rather extensive. The drawback to it is that because it is relatively inexpensive there is a fair share of people who aren't active but if you navigate around and find a good group of people who invite you into a private league you can have a good time.
The site is uber user-friendly and the bells and whistles are added more and more every year.
I do try to stay away from the free sites because they tend to be overloaded with trolls and dead-beat owners. Just a suggestion.
If you don't mind spending the cash, I've heard that ESPN.com is top-notch.