I run a Spesh Tricross singlespeed. Heavy-ish ally frame, fat 32 tyres with knobbles, extra brake levers on the bars, shallow drops, 42/18. Great hack, pretty cheap (£275 new), flip flop hub but fixie scares me shitless and smashes me bollox when I pull away so use SS now.
Only real problem is awful toeslap. Apart from that it's a blast and I can do no handed again like a nine year old. Hooray!