My favorite lab is Mpix Pro. They have quick turnaround, great quality prints/products, and pretty terrific customer service.
Typically when customers add-on digital files to their orders, I use my customer printed DVDs (also from Mpix Pro). I prefer these over flash drives because I've had some people accidentally delete the images off the drive and need to have them reloaded. DVDs are harder to lose too :)
That being said - how ADORABLE are these new flash drives and boxes? I might order some just to look at!
For those of you that don't need to print on Fine Art paper with special coatings, I highly recommend their non-pro site Mpix. :)