Money Saving Mom has the info on how you can get 20 free prints at Walgreens now through April 7, 2010. Upload them online, use code 20FREE4X6B at checkout and pick in-store pickup to avoid shipping charges. She says you have to order 20 print exactly, no more, no less.
I've personally used Walgreens to print photos like this before and I've been impressed with the quality. Their photo associates are also usually really friendly. Anyway, when you place your order you can choose which store to pick them up at as it gives you the time they'll be ready at each store. Last time I did this I chose a store I don't usually go to because it was on the way to where we were going to eat dinner and would save me a trip out later.