I've been looking for something like this for a while now.
The best free option is Google Notifr by James Chen:
It does however have some issues with multiple accounts (no idea if it is relevant to you).
Another option would be using MailPlane, which does everything you need and more:
If you don't mind running a separate app for checking mail, I can only recommend it. I ended up using it myself, since I have to switch between different accounts pretty often.
Found some more options, so to sum it up we have got:
- Google Notifr: - support for multiple accounts and Google AppDomains.
- GmailStatus: - pretty basic, you can set the checking interval though. Works with AppDomains. Does not support multiple accounts.
- Notify: - as mentioned by dex. Free version comes with Google AppDomain support and multiple account support.
- Gee: - pretty basic as well, supports only one account, since it uses Atom feeds, it probably will work with AppDomain accounts too.
All of the above solutions have one common problem, trying to open the inbox of a particular account from the notifier will just open Gmail in your default browser. If you are currently logged in into another Gmail account, you will remain there until you explicitly sign out. At least for me it is a major nuisance.
And as an aside, there is actually a way to set the checking interval in Google Notifier. Click on the mail icon, then hold down Command and Option and click on Preferences. You will get a dialog box with two fields. Enter AutocheckInterval into the upper field and the interval in minutes into the lower one.