Screenr is a web-based screencasting application that makes sharing short screencasts painless.
Screenr is Twitter-centric but sports a high-degree of functionality even if you don't actively use Twitter. You can easily tweet your screencasts from Screenr—the default, unless you specify otherwise is to share your screencasts on Twitter. If you opt to not share the screencast on Twitter you can download it as an MP4, upload it to YouTube, embed the screencast, or simply share the URL via email or IM.
When you start the Screenr recording tool you can specify which region of the screen you want to record—it uses a simple drag-to-resize box—and what audio input, if any, you'll be using. Hit the record button to start and the big green Done button to end your recording.
Want a beefier screencasting tool? Check out the Hive Five on screencasting tools here. Screenr is free, requires a Twitter-account for login, and has a limit of 5 minutes per screencast. Know of another great web-based screencasting tool? Let's hear about it in the comments.