So far, there are several good Gmail Dashboard Widgets to choose from. I will list of these by my favorites in order of their usefulness. Hopefully we will see some updates in the near future for these releases.
1: Gmail Google's own Dashboard Widget which allows you to see your Inbox on the fly in your Dashboard, and the ability to click on message to go directly to your account. This is probably the most efficient widget for Gmail I have used.
2: GMail Checker This is a nice looking widget, much like Google's own Widget. It allows you to see your Inbox messages with brief summaries.
3: Gmail Inbox Another preview Widget with a slightly different look, showing your Inbox in a nice looking albeit large window. I have encountered very few bugs with this one.
3: Gmail Counter 1.4 This Widget allows you to keep track of the storage on your Gmail accounts.