I have been very impressed with the Fibre Flare Light. It is a highly visible light, viewable from all around as it is has a fibre optic core. The manufacturer claims it is visible from up to 300m.
I had several positive comments about it on the recent Reigate Cyclosportive, and I am certainly happy that it increases my visibility greatly, and regularly use it in the day time too.
It has clips at each end and comes with straps for those so can be attached in a number of ways. It will clip on the back of a cycle jersey, but I tend to use it either down my seat post or clipped across my panniers.