Light it up
Easy install and battery swap.
nothing so far.
If you cruise around town or commute to work via bike, you probably already have lights sorted out for your bike. If not, the Niterider TL5.0 is a good inexpensive option to alert those behind you that you exist. Some of the nice features about this Niterider is that it is easy to install, comes with a rubber shim that should help fit any size post, it uses a common battery size (AAA), and it has a number of different flashing modes that you can cycle through.
It's probably not the fanciest or toughest rear light out there, but at under $10, this has been a good purchase and adds a little peace of mind when sharing the road.