"Gmail Notifier" is a downloadable Windows application from Google that alerts Gmail users to the presence of new incoming messages. The app displays an icon in your system tray to let you know if you have unread Gmail messages, and shows you their subjects, senders and snippets — without your having to open a web browser. Link (Thanks, ritilan)
Update: BoingBoing reader Oscar Bartos says, "There's an freeware program for Macs that acts like the Gmail notifier, Gmailstatus. Personally I'm fond of Gmailto, which redirects "mailto:" links to Gmail. It's available in PC and Mac flavors: Link, and Link 2. " And BoingBoing reader Nate says, "This site has a Gmail notifier extension for Mozilla/Firefox: Link" And an anonymous reader points us to yet another Gmail helper app — Gcount.
Update 2: Hey, you know what I could really go for right now? Some Gmail helper apps! BoingBoing reader Mincus says, "Mark Lyon's Gmail Loader has more — Link. And there are still more at Gmailwiki.com"