iOS only: If you're a Vimeo fan, or want to watch something recommended by a friend, Vimeo's app works just fine. But it's the ability to shoot and edit video on your phone, iPod or iPad, that's really nifty — and for free, too.
Apple's own iMovie app is $6 in the App Store, and while it's by all means a skilful editor, Vimeo looks really intriguing, especially for the price. You can drop in clips and audio and line them up on a familiar timeline, add transitions, effects and titles, and tweak the volume of each bit. When you're done, you can upload to the crisp, capable Vimeo service, of course, but you can also save back to your Camera Roll for uploading wherever you wish.
Vimeo's app is a free download for iPhone (3GS or 4), iPod touch (4th generation), or the iPad 2. If you're a mobile iMovie fan, we welcome your comparisons in the comments.