At PHI in the Gulf of Mexico we use mostly the (formerly) Goodrich Traffic Alert System (TAS). It actively interrogates the transponders of nearby aircraft and shows those within six or ten miles depending on the type of display. In the S-76 I've found it to be very reliable and accurate with respect to altitude, range, and bearing. It's my favorite piece of safety equipment. I understand the price tag is in the neighborhood of $15,000 US.
-Stan-