I can't believe no one has referenced Skitch yet. If you haven't tried skitch out, you owe it to yourself to do so. Super easy to grab screenshots, resize / crop, add arrows and annotation, and post online.
1. Jing (jingproject.com) runs on both Windows and Mac, less slick, but actually has more features than Skitch (Jing can also screen record for a couple minutes.)
but
2. I don't think this tool, screenbin, quite maps to Skitch or Jing.
Grabup has a closer functionality to pastebin. It's Mac only. Basically it adds functionality to the standard screen capture keyboard shortcut. You make the grab and then grabup will upload the screenshot and put a url in your clipboard. So you can just paste a link to someone.
sounds cool. Its fresh stuff. I'm sure if its as useful as it sounds, someone will host a well known screenbin server instead of the current DIY model.