Some Sam's Clubs will do 20x30 for around 9-10 dollars...it's not high quality work...there is a small white strip on the sides...but since I use my sports photos as posters in the TV room and rotate them on a regular basis, it is a good cheap way to get them printed.

Costco is a good value for inexpensive large prints. For the larger sizes you order online, they fulfill at a lab somewhere, and it arrives in a tube in the mail a few days later.

I was professional printer have used this company and compared with much much more expensive services.

http://www.elcocolor.net

Specifically look at their poster special.

At 3 inches away, the grain is better on the 300 dollar prints, but for 30 bucks for a 30x40, I truly recommend them.

Two cavets:
First, you must buy two prints at a time minimum.

Second, if you are professional and have calibrated etc your images, make note of that in their "special instructions" section. Otherwise, they will "autocorrect/destroy" the images in the machine.

Otherwise, I like whcc.com for their very attentive customer service.

I use http://www.myphotopipe.com/ Their print quality is fantastic, their prices are competitive, and they frequently send out good coupon codes via email.

They have a simple upload mechanism for basic prints, and a java-based app called ROES for customers who have large orders or want to chose specific options.

I've printed personal photos, and large sized prints that I had hung in galleries.

I'll second Costco. For the professional or anal-retentive amature, even provide down-loadable profiles for the printers in specific stores.

Shutterfly has been great for me with large prints.

http://www.shutterfly.com/