Reelights are powered by induction magnets which attach to your spokes. The lights attach at the hub axle and the magnets power the light as the pass by the light.  They’re bright enough for city riding, and the version I stock comes with a capacitor that keeps the lights flashing while you’re stopped.  At $55 you won’t find a less expensive battery-free lighting solution.  The lights work with solid or quick release axles.

How bright are they?  In my opinion, these lights aren’t  bright enough to be used as primary lights on roads with very fast traffic.  I’ve used these lights as a backup system on a bike that didn’t have a dynamo, and considering they’re 1/4 of the price of the least expensive dynamo front wheel/headlight/tailight systems they’re worth considering.

Reelight 120L Set with Power Backup