It's somewhat more complicated than that. The BIS server supports IMAP, POP, certain Exchange accounts with Outlook Web Access, and anything else they particularly feel like supporting. BIS in and of itself has support for Yahoo and Hotmail in spite of the fact that neither give you free POP/IMAP access, through some kind of private agreement (presumably meaning they access it through POP/IMAP sockets that you're not allowed to use).
What specifics you get varies on your service provider. In the US, T-Mobile explicitly provides Yahoo access (seems to be IMAP IDLE). They say that they do not provide Hotmail access due to commercial issues, but Hotmail works fine on my Blackberry 8700g with T-Mobile, and I did not have to go through any tricks to enable it (for the most part, to enable an account on BIS, you enter in your e-mail address and your password, and it is zero configuration).
This is what I can verify works fine: Hotmail, Yahoo, AOL, GMail, Blackberry e-mail address (mine is of the form
xxx@tmo.blackberry.net).
This is what I can verify does
not work: Netzero.
This can vary, I guess, if you're not with T-Mobile.
With respect to MSN... T-Mobile provides a free app that does Yahoo, MSN, and AIM, but it uses SMS for each transmission, meaning it's a ripoff. If you go to eveek.com or the like, there are discussions of alternative apps, although I do not know of any sort of Adium best of all worlds free solution on Blackberries...