salliemae dot com
Try them first - don't know what rates are right now, but like the above poster, I got a very low rate 2% on my first one and 4% the 2nd time I consolidate (first for undergrad, then for master's)
With SallieMae if you do automatic withdrawal, they shave off a .25%, so my current rate is 3.75%, then after 3 years of "on time payment" which if you are doing automatic payment, you will do no problem, you get another 1% off. So in another 2.5 years my rate will be 2.75%
Good Luck