Ok, that is a big question so this is a summary only.
First up, from March 2012 mode s is not compulsory for VFR flight. Right now there is no plan to make mode s compulsory for VFR.
TCAS
A traffic collision avoidance system typically costing $10k plus fited to high end GA +. I have no direct experience but you can find more;
Traffic collision avoidance system - Wikipedia, the free encyclopedia
PCAS
Portable Collision Avoidance System. A proprietary system costing £400 for an entry level system. It is intended as an assist to see and avoid. Assisted see and avoid is said to be 8 times more effective. I have such a system and am very pleased with it.
FLARM ('flight alarm) is a system originally intended for gliders. It works on GPS location and contains collision avoidance algorithms. It is light and portable and ideal for the lighter end of GA.
FLARM - Wikipedia, the free encyclopedia
ADS-b
Automatic Dependent Surveillance-Broadcast
Automatic dependent surveillance-broadcast - Wikipedia, the free encyclopedia
In my opinion ADS-B is the future but the time frame for this is unclear. If you have a mode s transponder (other than a Garmin 328), and an approved panel mount GPS you can “do” ADS-B now. The problem is that the CAA will charge you a major mod fee to do this so 99% of suitably equipped aircraft do not have the wire connected. This system could give us a known traffic environment out to 40 miles assuming we have a suitable receiver like Power Flarm, which combines many of the above tecs into one box.
Big word of warning, the above devices is a big step forward but will only detect suitably equipped aircraft. Only about 50% of the things you are likely to bump into have the kit you need to see them. If you research just how bad see and avoid is you will appreciate that this is a big step forward!
I have been campaigning to raise awareness of collision avoidance for some time and have given a number of talks and written a article in Flyer. I will be “testing” Power Flarm in the near future when mode C support is added. If you want a chat through all this send me a PM.
Rod1