As you may or may not know, I used to work for Netscape and have been using the Thunderbird e-mail client for many years. Since I use Thunderbird for work e-mail, I have been looking for an alternative e-mail client for my non-work e-mail accounts. On the Mac, I just use in the built in e-mail client. But for Windows I have not been able to find a good option until now.
Check out the PostBox e-mail client. It is based off of Thunderbird so it will look familiar if you have used Thunderbird before. It has IMAP support and seem to work really well with at least GMail. The client cost $9.95 and is definitely worth a look. Versions are available for both Windows and Mac.