Aviary has introduced Falcon, a web based image editing tool. With Falcon, you can capture images and web pages from your browser and crop, resize, and mark them up. Mark up options include text, shapes, colors, and more.
The best part about this is that it is web based so there is no software to install. I found it easy to use and very fast and responsive. This is a great alternative to desktop based image editors, especially for schools, as students can use it from home also.
It is free and you can save your files to your free account or download them.
UPDATE - There is also an Aviary extension available for Firefox and Chrome. One click and you are off and screen capturing, editing, etc.