If you run either Gnome or KDE, then Network Manager already does this for you. also, on the command line, you can issue the command "iwlist scan" and find wireless networks that way.

If you want more information, or are looking for a tool more like NetStumbler, then I would usggest you check out the airodump-ng suite of tools (all CLI), and Kismet (server + nCurses client), and wellenreiter (most like NetStumbler).