Skitch is a great Mac-based tool for simple image grabbing and manipulation. Perfect for sticking annotated images (like the one above of my daughter during a recent museum visit!) into emails, reports, webpages, etc.
Skitch was developed by the fine folks at Plasq (check out their tool Comic Life also) and it can be downloaded via the Skitch website. They have a great intro video that you can watch to get a sense of what Skitch can do.