Hoversnap is a great little free program, you can set it up to auto-name files, and you can capture as many as you want, and it creates whatever file type you want, and you can also set it up to automagically size the files too.
Fraps is another program which will take screenshots. You can set it to take a screenshot every X seconds, options to save them as BMP's, JPG's and PNG's. Also it lets you record videos too. It's not free but it is a very handy tool.