Its basically a 'packet sniffing' mode for wireless adapters, where - instead of sending only the packets that are addressed to a particular adapter to the CPU, it sends ALL network packets to it where they can be analyzed aka. sniffed out. Programs like Kismet and AirSnort can use this data to monitor traffic across the whole LAN.
As far as adapters that support it... only one comes right to mind. I know that the AR5005GS supports 'promiscuous mode', however at this time I haven't found any Windows platform drivers that enable it. If you want to do your 'sniffing' under Linux however, you can download the drivers from SourceForge's MadWIFI site and they will allow you to enable it.
Here is a link to a site you could potentially get the card from...http://www.oxfordtec.com/us/p106/GIG...duct_info.html
Another site... little cheaper but shows out of stock...http://www.monstronix.com/gnwiag02-p-81256.html
Anyway, looks like Linux is where the sniffer programs shine anyway, so you could always get this card and run a dual boot Linux/XP or Vista type setup and have the best of both worlds. In any situation, the Atheros card will blow away the performance of the stock network card in your 9300 anyway.