Mac OS X: Skitch is our favourite screen capture tool on the Mac and although it's free, there are a lot of neat features you get with the paid version. Currently, if you purchase it on the Mac App Store you can get those features for half off.
Bear with me because this is a little confusing. Skitch Plus regularly costs you $US19.95 per year and comes with several additional features like capturing full-length web pages, watermarking images, additional file formats, no ads and much more. If you buy it on the Mac App Store you pay $13 once for those features and get basic Skitch.com sharing. What you don't get are the web-side extras that come with a premium Skitch.com subscription. So for half the cost that you only have to pay once, you get all the premium software features but not the Skitch.com features. If you've been looking to upgrade, this is a pretty good deal.
Skitch (Plus) [Mac App Store]