I just switched to Burrell (burrellprolabs.com) from White House (whcc.com) on a recommendation from another photographer. I think White House does a great job, but I felt the quality was lacking a little on a large print they did for me. I just got my first two 16x20s back from Burrell, and they are gorgeous! I'm still using White House - they've got a great deal on photo business cards - I just got 1000 of them. I ordered a set of greeting cards from White House for an image of my son that I hand-colored. I had it printed on watercolor paper - absolutely beautiful!
Both companies only work with professionals (Burrell said this right out - asking for my tax id #, WHCC implied it in their questionnaire), but you sound like you're heading that way, if you're not already there. I hadn't acutally made a sale as a professional when I signed up with WHCC, but didn't have any problems getting set up (the second time I filled out their form - I got no response the first time). I had only a couple of sales under my belt when I signed up with Burrell (only a few weeks ago) but had no problem getting a customer account set up. A very friendly customer service rep called and took time to set me up and explain everything to me.
Both labs impressed me by offering free color calibration prints. With White House, you email five pictures, and they print them 8x10 free of charge. With Burrell, you send three pictures, and they print them as you sent them, and color corrected - a total of six 8x10s. This is so you can decide whether to order with their as-is service or their color-corrected service.
WHCC lists its prices online in a pdf, Burrell doesn't list theirs online. You can download their software for free, though (Print it Now for as-is, Print it Pro for color-correction) and you can see prices that way. I expected Burrell to be a lot more expensive, but they turned out to be only slightly higher ($.10 more on an 8x10) using the as-is option.
You put "web hosting choices" in your subject - do you have a question on that as well?
I use MPIX with little to no problem. I like the fact that I can order JUST the prints I want using their online viewing system. Email me (firstname.lastname@example.org) and I can send you a link to a picture of two from there so you can see what it's like if you want.
I use Mpix. I burn the cd and send it in free postyage, btw. Make the selection on line and the prints are back speedily and match what was on my screen. About one week from th time I put the cd in the mil and have prints in my hand.
If yoiu have a volume of work and want a company to manage the sale and shipping of prints, look at programs like that from outfits like Pictage.
These fine folks have covered the online labs pretty well but I do have some thoughts.
Before you choose an online lab you need to determine how you intend to use the lab and how any particular labs capabilities fit into that plan. Some want a lab for nothing but prints and handle everything else themselves, others prefer a lab that offers full service including web hosting the online proofs, taking print orders, making and shipping prints etc.
Me I still use my local pro lab and I do not see anytime soon that I will be changing. Even with digital I just prefer the ability to turn my work over to a lab tech that I have been working with for many years,