Natively written packet sniffer / inspector for OS X
Packet Peeper is a free, native OS X application for packet sniffing. Not quite as robust as Wireshark (but what is really).

There's also Cocoa Packet Analyzer, which appears to have a nifty plugin framework for extending its capabilities, but I've never used it. It's donation-ware.

KisMAC may also be of interest. Not so much a sniffer as a wireless stumbling tool. I've used it successfully to tune my wireless network in a congested neighbourhood.

Edit: based on your updated desired features I'd say try Packet Peeper. It fulfills both your requirements.