Linux: Ubuntu 9.04's Growl-like, transparent notifications are slick and convenient for corner-of-your-view updates. Gmail Notifier takes full advantage of the latest Ubuntu improvements to provide lightweight Gmail checking on a schedule.
There's not a lot more to say about it, other than it requires only your Gmail username and password, and it seems to support Google Apps email addresses. You set a number of seconds between checks (it politely asks for a minimum of 3 minutes between updates), and a fade-in window brings the new email counts and subjects to your desktop. There's a panel icon that also lights up on new messages, but I removed it from my top panel because it didn't un-light until the next check, and I like to keep my email checking at a 30-minute interval, if not longer.
Grab a package at the link below, unpack it, and open the README file for installation instructions and dependency requirements. If you're looking for more Gmail-checking features and don't necessarily need total desktop integration, try the previously covered cGmail. Gmail Notifier is a free download, built for Ubuntu Linux 9.04 only. Thanks Marc! Highly Ubuntu 9.04 integrated GMail Notifier in Launchpad [via Kabatology]