Fancy Free, the writer's Schock 35 recently added an AIS receiver to complement her radar system. It is rare to find a product that exceeds ones expectations and is of great value, AIS fits this category.
A ship equipped with an AIS transponder can broadcast a great deal of information, getting as detailed as their draft and rate of turn. Of primary interest to sailors, and overnight racers in particular are the position, speed, and type of vessel.

AIS was implemented by the US Department of Homeland Security/Coast Guard, their website lists which vessels must broadcast information, they include self propelled vessels over 65' in commercial service, towing vessels over 26' with 600hp or more, all tankers, passenger vessels of 150 gross tons, other vessels over 300 gross tonnage. Fishing and pleasure boats are excluded.
To receive the signals your yacht will need a VHF antennae, you can either have a dedicated AIS antennae or use your existing VHF one with a splitter (and suffer some degradation to both signals), the receiver itself, and software that will interpret the information.
On the Schock we went with a dedicated antennae, a receiver ($189) purchased online from Milltech Marine (you can find more expensive receivers with prettier colours elsewhere, try the boat shows), and Coastal Explorer navigation software.
A bundled package of hardware and software is available from Milltech for $500. Adding antennae and a serial to USB converter and taxes would be around $600. Ship it to the Letter Carrier in Point Roberts, pay the taxes at the border and you're done.
Coastal Explorer also has a 10 minute demo version available which can be downloaded, however in the middle of the night when a cruise ship is bearing down, 10 minutes seems very short and $369 very cheap.
A really nice feature of Coastal Explorer is its ability to search the computer ports (I guess you need a laptop too) and identify devices such as GPS, AIS & Magnetic Heading. With your own position and heading the software can then compute "closest point of approach" (CPA) and "time to CPA" allowing you to see if there is a risk of collision when you select a vessel of interest.
You can also project a time vector (assuming constant speed) from your vessel to see if you are going to pass ahead or astern and provide you with an idea of how tight the crossing will be.
Range is dependent on VHF reception, however moored near Granville Island we can pick up and identify all of the transmitting vessels in Burrard Inlet and as far south as Roberts Bank Coal Port. Obviously radar would never come close. I long to get rid of the weight and windage of the radar, but will keep it for the occasional fish boat in the fog.