This works as following: you upload some document, for example, a .gif picture, choose a format you want it in, for example .jpeg, and send you e-mail. Then, you get a confirmation that your document has been converted and you download it back to your computer.
Zamzar currently supports the following format conversions. The list is pretty long, although we could remember a few formats that it doesn't support, for example Matroska files (.mka). Still, it's a really useful service, especially for those less tech-savvy users who don't want to spend hours on Google (ok, it's minutes actually) figuring how to do this themselves.