I opted for the light on the picture below. It is powered by 2 x AA batteries (included). It is very light and has the traditional 3 x modes, of just on and two flashing. The 5 x LED's are very bright and certainly allow one to be seen.
The light can be mounted horizontally or vertically which is great for the rear seat pillar on a Brompton and you also get a clip mount, which allows you to attach it to a bag or jacket.
I bought mine from Evans with a London Cycling Campaign discount for £15.99. There are of course more expensive lights out there but I didn't really want to pay more than £20.