If the 457 plan is the only one your company offers, the limits are the same as with a 401(k) - a maximum of $16,500 in 2010 for those under 50 years old, and up to $22,000 for those 50 and over.
But here's the difference: If your employer also offers a 401(k) or 403(b) plan, you can contribute to both the 457 and the other plan. Moreover, you can invest up to the maximum in each account. In 2010, the limits are $16,500 in each type of account, plus catch-up contributions - so you could make a total retirement contribution of as much as $33,000 (or $44,000 if you are 50 or older). Investing the max in both is a terrific option if you're getting started saving a little late, or you just want to maximize the advantages of these plans (tax breaks and matching, if any).