There is a widget available - informer - that has POP3 checking and a lot of other good stuff; best widget I've seen. Try it - you'll like it.

Widget still works for me. Let me know what specifically isn't working for people and I'll have it fixed. This has really been a side project for me but I want to get it back working very soon and would like to fix any specific problems people have. Let me know ASAP and i'll get it fixed, hopefully.

Unfortunately this widget no longer works.

this doesn't work smoothly if your email client is set up to leave messages on the server for a period of time. every time MailMan checks the server, it pops up if there are any messages there at all (as opposed to just new ones). this defeats the purpose as it will ALWAYS say i have messages, when in fact they're not new.
i leave messages on the server for a week so that i can reply to previously-read messages while on the road. if the client deleted them on download, i wouldn't be …able to access them remotely. instead, Thunderbird deletes them a week later.
don't know how the widget's code works, but it seems a simple fix. rather than looking for a msg count >0, why not just keep track of the msg count since the last client launch, and then pop up when the number is greater than that? More

I just updated to Konfabulator 2.0.1 and this widget stopped working. It's a shame too, because it's the most useful out there...