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What's your favourite? I've just bought one of these:
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12 hours of bright flashing light for street-lit city/town riding. Self-contained rechargeable unit via PC USB port.
Philips Ridesafe - single unit with shaped light beam (thank you, German lighting regs), 80 lumens , USB rechargeable, but will take 4 regular AAs. Some good reviews (Google) & ideal for unlit commutes.
My favourite (since it's the one I've got) is a Trek Ion Recharge. Light, relatively cheap, good bracket, good enough beam and runtime. Only niggle is that the flash could be a bit quicker.
I have two Blackburn Flea Lights. Really compact but only good for increasing visibility under street lights...
Tons of 'em. Cateye seem excellent for being seen- one of those on flash, one on solid. An old tesco cree torch on the bars which is useless for being seen, but decent for seeing. And an XML torch on the helmet, which is ace for seeing, and has the capability to grab people's attention.
dx jobber with a persil washing liquid pot for a 'glow ring'.