Windows Live Mail
Have you tried this? If you are familiar with Outlook, then you will find that all the rich features of Outlook (like Rich Text Signatures, RSS Feeds, Newsgroups, Multiple POP/IMAP account configurations) can be done with the help of this client. You can configure Live Mail to receive mails from GMail, Hotmail and Yahoo! (provided you have Yahoo! Plus account).
Quite a handy app considering that the interface provides me an opportunity to send mails using multiple accounts and also has separate folders for the different email accounts that I configure it for.