As part of my search for work in Ann Arbor, I've been compiling a list of employers who maintain lists of open opportunities on their web sites. You can see the list here
As of this writing there are about 60 employers listed, though not all of them have open positions at this time.
There's a wide variety of online job postings systems being used by employers, ranging from the simple web page listing one or two positions to the unimaginably complex setup when an HR organization has hundreds of positions to fill worldwide.
Quite a few positions are filled without going through online job boards, of course – but it never hurts to see what's posted and available.