I'm not a fan of most crowdsourcing sites. More often than not, they're a trap for novice designers, a pyrrhic victory for clients, and they lower the bar for good design and fair pricing everywhere. (Interesting discussion and articles on the matter.)
But...Minted is different. Simply put, it's like Threadless for invitations. Minted attracts some great designers from all over the world (but mostly the US), who submit stationery and invitation designs based on each month's competition guidelines. They're reputable, honest, and their assortment is a pleasure to browse. Winning designs net cash prizes, as well as a 5% commission on each design sold. Pretty dang sweet. Plus, as a semi-obsessed design submitter, it's a lot of fun.