If you want full Exchange services support (LDAP), a company called Bynari makes a client that supposedly supports all of them. I've never personally used the client, but you can find more info about it here:
Woops, correction. Just did a quick look and it seems Bynari has made the client open source. It's called TradeClient and can be found here:
http://tradeclient.sourceforge.net/
Cheers