Gmail !
Yes ! I have a gmail account now too..
Although the primary utiliy that I thought it had was that of the 1 GB space , I realize that it has a very cool interface too, many "bells and whistles" put in there that makes it a useful tool..
Some that come to mind are :
cool key board short cuts , messages are grouped together as conversations and not as individual messages , a very very cool autocomplete feature for email addresses ...
Me and my colleagues at work think its almost a different paradigm , for example you never need a ftp server now ..as you can store all your important files just on google mail (have to stress test with file sizes though )..we also did a back of the envelope kind of calculation whether the space is a big issue ..turns out it isnt..