Other than snort?
Dragon (unix only last time I checked)
ISS Real Secure (can't this use snort these days?)
Cisco Netranger(what is it called now?)
prelude-ids (uses snort rules)
They all suck to varying degrees.
Quick google search: Computer Associate International's eTrust, Cisco Systems' Secure IDS, CyberSafe Corp.'s Centrax, Enterasys Networks' Dragon, Internet Security Systems' BlackICE ISS' RealSecure, Intrusion.com's SecureNet Pro, NFR Security's NFR Network Intrusion Detection System, Anzen Computing's Flight Jacket, open-source Snort, Symantec Corp.'s NetProwler.