For stealing Instant Messages off the wire (if you're using a hub, ARP-spoofing, or you're inline):
http://freshmeat.net/projects/chatsniff/?branch_id=59291&release_id=203759
Supports AIM/ICQ, MSN, Yahoo!, and Jabber
Is this only for little networks, or could I use it on you for example?
As I said, you have to be on the same computer, on the same hub as the computer, or inline (ie, their connection passes through your computer)
Like... when you're stealing wireless internet and feel nosy ?
You'd still need ARP Poisoning, since wireless (I think) counts as a switch.