Depends on your budget. Countryman, Sansamp, BSS Audio, and Avalon are used a lot in studios. If you want to spend $500 look at the latest Bass Player magazine for a review of the Avalon.
Whirlwind, Boss, and Behringer make some good inexpensive ones. Under $100. They either run on batteries, phantom power or both.
The Bass Pod can be used as a direct box and it's very versatile for around $300 dollars.