PopMan!

POP Peeper is an email notifier that runs in your Windows task bar and alerts you when you have new email on your POP3, IMAP (with IDLE support), Hotmail\MSN\LiveMail, Yahoo, GMail, Mail.com, MyWay, Excite, iWon, Lycos.com, RediffMail, Juno and NetZero accounts. IMAP supports allows you to access AOL, AIM, Netscape and other services. Send mail directly from POP Peeper and use the address book to email your frequently used contacts. POP Peeper allows you to view messages using HTML or you can choose to safely view all messages in rich or plain text. »

nPOPuk is an ultra-small, but comprehensive, e-mail client that runs on a wide range of Windows-based operating systems

Mia is a native Gmail application on your Mac's menu bar!

GMDesk is an application that lets you run Gmail, Google Calendar, Google Docs and Google Maps as a stand-alone application to do all your mail handling, calendar event reading etc with. No need to open it up in a web browser, or have it clutter your workspace.

This is a Firefox extension that allows you to manage multiple Gmail accounts and receive new mail notifications. It displays account details, including unread messages, saved drafts, spam messages, labels with new mail, space used, and new mail snippets.

Fastest Gmail is a handy and reliable extension designed to notify you about incoming emails from all your Google Mail accounts and labels.

Gmail Notifier for Windows 7 monitors your Gmail inbox and notifies you of new emails. It takes advantage of the latest Windows 7 UI elements such as the taskbar overlay icon, jump lists and thumbnail previews. It's fully compatable with Google Apps accounts too.

Gdow 2 is a free Gmail notifier with shareware "on the fly" inbox management capabilities.Gdow 2 uses atom feed parsing for notification and simplified Gdow 2007 engine to connect and interact with Gmail account.For now interaction includes basic "on the fly" actions : delete, spam and "mark as read".