small computer (central Ann Arbor)

Hi,
I'm looking for a very small-sized desktop computer. Hewlett-Packard makes something called a "small form factor", for example, and that's the size I'm looking for. Or smaller, that would be okay, too. I'm not looking for a super powerful computer, just something that can be used to go online for email or for writing short documents.
I don't need a monitor or mouse or keyboard, just the computer. It doesn't even need to have an operating system as long as it powers on and is usable.
Thanks