Screenr is a very simple, easy-to-use tool for creating screencast videos. You do not need to register in order to use Screenr, but if you want to save your recordings you do need a Twitter account. Screenr uses your Twitter ID to save your recording and publish it to Twitter (you can opt not to publish to Twitter). The recordings you make using Screenr can also be published to YouTube or you can download your recordings. Screenr is my favorite of the tools on this list.
Screen Castle is a simple screencast creation tool that is completely web-based. To use Screen Castle simply visit their website, click the start button and you're recording. You have the option to enable voice recording for your screencasts. Screencasts made using Screen Castle can be viewed on the Screen Castle website (see my example here) or embedded into another website or blog.
Screencast-O-Matic is a web-based screencast creation tool similar to Screen Castle. Screencast-O-Matic allows you to specify how much of your screen that you want to record. Recording your voice is an available option. Every time that I've tried Screencast-O-Matic it was slow so you probably need to have a fast Internet connection to use Screencast-O-Matic effectively.