I've heard wellenrieter is good, but I think kismet is worth at least a little extra effort to get it running. If I have it right, kismet is a passive sniffer which means you're invisible, whereas wellenrieter is like netstumbler which sends out probe request to see who responds. If a wireless router doesn't respond then netstumbler/wellenrieter won't find it. Correct me if I'm wrong.