Random Gmail Tips
A few people have been amazed when they saw me do this lately, so I guess it's not common knowledge: you can enable keyboard navigation in Gmail. It lets you scroll around messages, tag things, archive things, etc without using the mouse, and it's awesome. Official Instructions for Gmail Keyboard Navigation.
Also, I recently discovered the It's All Text addon for Firefox. It lets you optionally edit any textarea in your favourite text editor - and that includes Gmail. Very cool if you're sick and tired of proportionally-spaced blurry textarea fonts that make "i" look almost exactly like "l".