Just finished reading this article:
Heidi M. Steiner, (2010) "Livening virtual reference with screencasting and screen sharing", Library Hi Tech News, Vol. 27 Iss: 4/5, pp. 9-11. DOI 10.1108/07419051011083172
Heidi takes a look at Jing, Screencast-o-matic, Screencastle, and Screenjelly for screencasting, and Dimdim, Yuuguu and ShowMyPC as screen sharing tools. Missing from her list is a new tool I haven't taken the time to explore yet either, join.me (oh hey, I just noticed that site's run by logmein.com, which I love!). She doesn't make an overall recommendation, just briefly lists the pros and cons of each tool. If they're new to you, it's a nice annotated list.
Scott Leslie was also recently testing, and reports his findings in Quick Shoot-Out – 4 Free Web-based Screencasting Tools. He covers three of Heidi's tools, and also ScreenToaster. Scott picks Screencast-o-matic as his winner, but proclaims web-based screencasting "now mostly “Mom-proof” technology."
If you're looking for quick and easy screencasting, you'll do well with any of the above.