I think the laws are different depending on the state but most of those overly broad agreements are not enforceable. Usually it has to be something that you could have only created with proprietary knowledge gained from your employer or access to customers gained during the course of business. So just because you are a web developer by day, your employers do not own the Minecraft forum you maintain by night.
I'm not a lawyer, so this is just my fuzzy recollection of what my lawyers told me. If you've got something serious on the line, a good lawyer's time is a cheap investment that will pay dividends in the long run.
