There is a device made for gliders and light aircraft which gives warnings of proximate traffic, is cheap and requires very little power. It also has an obstacle database.
http://www.flarm.com
It has a high take-up rate in the alpine regions of Europe and is mandatory for many competitions in this area. It doesn't interact with current ATC or TCAS installations, however.
I've got a mode-S transponder fitted to my glider but I do have 40Ah of batteries plus solar panels as the current drain can be substantial in a busy environment. I'm interested to see how someone can produce a 'low power' XPDR and stay within the SSR specifications for output...
If you don't know, the CAA are keen on mandating the carriage of mode-S on ALL aircraft in ALL airspace in the UK, as opposed to the rest of Europe where it is only required in certain classes that are usually close to major airports. The consultation period for this has just closed, so we'll have to wait and see now. The cost of installing, checking, licensing and maintaining mode-S kit could run to almost the value of some of the gliders it would have to be fitted to, not to mention the problems of getting an adequate power supply onboard.