I also like moo.com, but they are on the higher end. most mass printers are going to gang print your work and therefore only generally color correct. I do a fair bit of offset printing so for what it's worth here are my thoughts on a few places
40ver.com - to the trades only, best combo of pretty good color mgt and price - nice thick postcard stock too. nice options like round corners on more products
printrunner.com - similar experience to 4over, nicer people, more expensive. fast turnaround, great customer service
gotprint.net - lots more expensive lots better color (still gang printed) - more options
vistaprint - worst of the cheapest, crappy paper, lousy color AND resolution control, dirt cheap.
blossom publishing - lots more options for a mass printer, WAY more expensive, they're a photographer marketing kind of place like Marathon and if you need design services or just don't know anything about offset printing they'll do a great job but the same thing can be had for a lot less.
there are a bajillion others, I've found that there's a sweet spot in price+paper quality+ ability to get my file close to what I wanted in a gang environment.