I think someone is using my Internet.

Obviously putting a password on it will stop them, but what about the potential damage that could have already been done? They are obviously download something, because they're sucking up all my bandwidth. What if they're downloading something illegal and I get into trouble for it? Can I call my ISP and find out who it is?

umm technically they could access whatever computers on the network if theyre unsecured. If you have a bandwidth cap too they could help screw that over too

Seriously, just put a password on you're wireless router, and make sure it's WPA or WPA2. That will probably stop them from ever going on your internet again.

Best thing to do is just encrypt it and hope nothing happens, it's very unlikely you'd get caught out for someone downloading on your connection. Unless your router keeps a track of all MAC addresses that have recently connected to your wireless connection and then you go around checking people's laptops, there's no way you'll know who was connected. Also, are you sure someone was connected at all? Could have just been your internet being a bit crappy.

" Obviously putting a password on it will stop them, but what about the potential damage that could have already been done? They are obviously download something, because they're sucking up all my bandwidth. What if they're downloading something illegal and I get into trouble for it? Can I call my ISP and find out who it is? "

Download McAfee Total Protection trial and it will tell you what computers have access to your network and you can stop them from having any access.  I did this just a few days ago and found that 8 computers were actually on my network so I blocked all of them. 
 Here's where you can find it:
http://home.mcafee.com/Store/Downloads.aspx

just set a WPA password and the problem is solved. 

They probably already downloaded a billion gigs of child porn, animal sex, bomb making manuals, Yogibearxvid.avi and some music albums.
 
Might as well sell the house and go live somewhere else with a new name.

Chances are they won't have done anything with your connection that could get you in trouble.

your network is not password protected? Why not?

" They probably already downloaded a billion gigs of child porn, animal sex, bomb making manuals, Yogibearxvid.avi and some music albums.  Might as well sell the house and go live somewhere else with a new name. "

close the curtains and get a dog

There are people who don't password protect their Wifi still? 

Need to protect that shit; other than that, might be hard to find out who is scamming your net.

" There are people who don't password protect their Wifi still?  "

it's totally not me...