Skitch is a great screen-capture tool for mac, which allows you to annotate your captures.
The newer version of Skitch (> 2) seems to require an Evernote account. I don't know what Evernote is, I don't want an account, and I don't want to be reminded that my "password expired".
My solution has been to download the latest 1.x version of Skitch from http://www.oldapps.com/mac/skitch_for_mac.php?old_skitch=10010, and voilà, no more annoyances.