It’s still in Beta but you can now sign up and don’t need the invitation from another Gmail user. I 
(love) Gmail.
It’s great on the fone (m.gmail.com) or you can download the app for the Blackberry. I also like, no wait, probably love, the search functionality, it actually works and the filtering is not at all difficult yet very effective.
It is also really good at filtering spam and has a built in chat module that you can also install on smart fones and chat to twitter.com!!!
PS. the 2835 megabytes* of space is pretty cool (as of writing this, it is always increasing).