There are a few websites that will take your old cell phones but the most popular one is cellforcash.com. The good thing about doing this online is that you can find out how much they are going to give you for them before you send them.
As the above poster wrote, you can donate them and thats always a nice thing to do. Plus if you own your own business you can use that as a tax write-off. Just get a receipt when you drop them off.
Good Luck