SMART Halogens, £30 on Wiggle. Two lamps (spot and flood) and plenty bright enough - I had cars flashing at me to dip! - plus good run time. Big battery though but for cheap commuting they are ace.
Alternatively I got some Dinottes from the US, not cheap at £90 (but good compared to the £150 if you get them in this country) - super light/minimal, the youthful grease monkey at the LBS thought they were 'trick' (I think this is a good thing?), plenty bright enough on dark country lanes and swapping between bikes takes about 30 seconds which is not the case with the SMARTs.