Flashlights are one of the more useful things to have in Davis, especially considering the Light Ordinance can make for pretty dim ambient light in places like Old North Davis. You can't be too careful when the light goes away. If you ride a bike at night, you need light. A flashlight is a good source of this (as opposed to a dedicated bike light) if you have the proper mounts. As if you needed another reason to have a flashlight, the motto of the university is fiat lux.
Biking with a flashlight
Most of the bike lights you'll see around town are cheap plastic deals that are dim and fragile. After you work your way through several of these, you might realize that a good LED flashlight would probably be more durable and flexible, especially with the appropriate mounting. WilliamLewis recommends the Fenix L2D CE mounted with TwoFish lockblocks, which should put you back about $55.
Where to buy them
Unfortunately, the best flashlights to be found for sale in this town are likely some LED-based ones from Inova, available at Ace Hardware. These are leaps and bounds ahead of the plastic cheapies that can be found everywhere else, but not much of an improvement over Maglites.