good, free, open source email client with IMAP and POP

You might try checking out http://www.pmail.com - I'm not sure about the IMAP, although I THINK it's in there, but it's definitely free.

DrJohnEvans: my main problems have to do with folders, not being able to rename/delete them, etc.,mostly in FB. In the bat, I can see the folders and message headers, but not message bodies (though i suspect I'm doing something wrong). Have Mozilla installed but haven't tested it extensively.

Thunderbird IMAP has a few minor flaws, especially in the performance area, but it really does work well for reading. Its ability to organize folders is, admittedly, not too hot. KDE's http://kmail.kde.org/ is a little goofy to configure for IMAP but it's been working great for me. I like it more than Thunderbird because it's actively hostile towards HTML messages. And yes, KDE http://sourceforge.net/projects/kde-cygwin/, if that's your OS of choice.

Why can't you rename/delete the folders? The options are in the right-click menus. Did they just not work? I'll second DrJohnEvans' endorsement of Mozilla's IMAP; I've been using it for about as long on three to four accounts with lots of folders and have never been let down.

Also far be it from me to let a mail client thread go by without mentioning http://www.mutt.org/, pretty much the best MUA of all time, which does indeed http://www.geocities.com/win32mutt/win32.html as well.

Yeah, Mutt rocks. The level of configurability is amazing.

signal: Have you used The Bat 2.0? The 1.X IMAP implementation was terrible, but the main addition to 2.X was full IMAP support.

Mozilla has served me well for a long time now, and I have 100 or so IMAP folders, sometimes nested as deep as 5 levels.

I can't seem to nest folders using Thunderbird -- would that be its fault, or the IMAP server's fault?