Since getting a Mac, I’ve become a fan of the graphics editor Skitch. It’s a simple and useful application that I’ve been using more and more as of late.
Skitch is great for taking a quick screenshot and resizing it quickly. You can press a keyboard shortcut to take a shot of the entire screen or a selection. You can then crop and resize the image by dragging parts of the window border, then save it to your hard disk or a remote server (perhaps your blog’s uploads folder?). You can also annotate the image with text and arrows, add a drop shadow to the resulting image, etc.
I find Skitch useful for blogging especially, since I often snap screenshots for posts. I also use it to resize existing images when it’s too much bother to launch Photoshop.
If you have a Mac, it’s definitely worth checking out this free application. There isn’t a version for Windows, though I would like it if the folks at Plasq Software would write one.