Good question, I would like to know this too. Currently I just use Bandwidth Monitor Pro, I find this program great to see if there is bandwidth being used for unknown reasons.

Most wireless routers have a display in the web admin interface that shows who is connected (often along with IP and mac address).

meh, ok that's easier than expected then. So you don't need anything special to find out who is using your wireless internet...it just tells you? and if it doesn't...what could you use?

I didn't know that, thanks for explaining it Jason, I have wireless and try to stay on top of keeping it as secure as I can.

No.. just be aware that some thieves may actually steal/copy mac addresses and ips. Some routers allow you to limit ip and mac address access, so they will often catch these in the clear and then use them on their system.

Yes...some people are just bent on destruction :/

Although, if you see someone using your internet connection, don't panic, it may just be one of those person that is using it for non-destructive purposes.