Windows/Linux: Free, open source application GmailAssistant monitors multiple Gmail accounts and alerts you of new mail. The application can monitor anything from your inbox to specific labels, so you can get very granular about what you want to be notified of. The biggest drawback is that GmailAssistant does not support Google Apps on your domain, but if you've only got Gmail.com accounts, that won't matter. Aside from that, GmailAssistant is highly customizable for individual accounts and works like a charm. GmailAssistant is free, Windows and Linux only, requires Java. If you prefer to keep your alerts in the browser or you want support for Google Apps addresses, check out the Gmail Manager Firefox extension.
GmailAssistant [SourceForge via Download Squad]
