Mr. Thinner. But I think it's mostly isopropyl alcohol.
I used to use any alternative thinner I could get my hands on--mineral spirits from the hardware store, windex, windscreen wiper fluid, denatured alcohol, lacquer thinner, distilled water, etc. Found out that using the manufacturer's thinner really wasn't much more expensive, and the paint just feels like it comes out better.
If you just use the manufacturer's thinners for strictly thinning the paint, and all the third-party stuff for cleaning up, the expensive stuff goes a *long* way. I've got a big bottle of the Mr. Thinner (bought for around $8 IIRC), and I have yet to get to the 3/4 full mark--and it's been about 3 months since I got it.
HTH,
Dan