I normally use Walgreens. I upload my edited photos to their website and have them printed at my local store. For friends and relatives I have had them mail the photos. This is very convenient but costs a little bit more.
I would guess that lots of retailers have similar services. There are also internet businesses that specialize in printing photos and mailing them to you. There are a lot of things you can do beyond just getting 4x6 prints, such as large prints, odd shaped prints, matted prints, prints on tshirts and coffee mugs, even bound books assembled from your prints.