Jing is always running on my computers.  It’s my go-to tool for screenshots and screencasting on both my PC and Mac.  It’s a free download that lets you select any portion (or all) of your screen to record as an image, or a video (up to 5 minutes).

You can enhance your screenshots by drawing, typing, or highlighting on them.  Screencasts can be saved to your computer or uploaded to Screencast.com (also free).  You can even embed your material from Screencast.com in your web page.