I've used cbox in multiple ESPN leagues for two years now and it works with no problems. You can customize the look and size from their site once you log on (it's more flexible than I first thought). Additionally, for a one-time fee, you can buy the advanced version with some extras (I did that for one league where people were posting as other people, so the upgrade lets you create an id/password for each poster so you know who's posting regarding of the stated name).
I don't know if other services offer more, but cbox is free, basic and flexible. I definitely recommend it for the day-to-day chatting while using the message posts for the larger/more serious issues/official issues and threads to discuss.