Android only: For all the widgets that Android comes with, Gmail is oddly absent. Free app GmailWidgetPlus will display your inbox's unread count, organised by label, on your home screen.
Not everyone needs menu bar notifications for each Gmail label, but Android's Gmail app does an especially bad job of letting you know when you have new non-inbox email. To see a list of labels and their unread counts, you have to manually hit the Menu button and go to "Labels" — not very convenient.
GmailWidgetPlus is simple, but fixes this annoyance nicely. When adding it to your home screen, you can choose which account acts as the default, and whether to show all labels or just the ones with unread messages. There are even two different skins you can apply, to make it look nice with all your other home screen widgets. It won't notify you through the menu bar, but it's nice for periodically seeing when you have unread, but messages that aren't imperative in your email, such as newsletters or Facebook friend requests. Hit the link to check it out.
GmailWidgetPlus is a free download for Android devices.