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.
Sanket Sonavane Senior Developer, IoT Enthusiast, Photographer, Bike Rider, Chef, AndroidUser, MacOSX Fanatic ;) Loves computers, programming, web developement, micro-controllers, gadgets, automation and many more such tech innovations that defines the way we live in the modern age and likes to explore what more can be done to help the human race :)