I learned today that Gmail offers free IMAP access now (they have for awhile, but I'm out of the loop, I guess). I wub Gmail in general. I forward all of my email accounts to my main Gmail account--except for my university account, which is separate. Using Gmail for their email system was, incidentally, the smartest technical decision my university ever made.
However, up until tonight, I've been using POP to view my messages in Thunderbird. For those of you unfamiliar with mail protocols, POP lets you download messages off a server, but you can't really interact with them. This means that if I download a message off Gmail in Windows, for example, then when I go to Kubuntu and open up Thunderbird there, the message will have already been marked read and sent to Gmail's archive. There are various settings I could have played with, but in the end, IMAP is the way to go.
IMAP, unlike POP, keeps the message on the server and lets you modify it on the server. Now that Thunderbird accesses my accounts through IMAP, I have access to every single message in my Gmail account--not just the ones I've downloaded onto my computer. My folders and filters can sync up now, so if I move something through Thunderbird, it moves in Gmail too. That means that if I have to login in from another computer, my email's all organized in Gmail.
I'm quite excited to be using IMAP with Gmail. Thank you Google. You have my vote if you ever try to take over the world (or is this a sign that you're already doing that?). :D
Update: This article at LifeHacker was a big help in getting the most out of Thunderbird/Gmail/IMAP.