Here are the covers and samples of some doujins that aren't in this site. If you have or find any of these, then please upload it. Also, check the Bounties section up there, some doujins have a juicy reward if you find them.
Please note that some names MIGHT be wrong.
I can't keep track of all these doujins, so if you find that any of these is (or was) added send me a PM. Thanks.
EDIT: Two doujins were removed, one was already on the site (thanks SaittaMicus) and the other one was uploaded by begbeg
#602 may have additional content so I'll leave it here
#623 Mugimugi says it's from an artbook http://doujinshi.mugimugi.org/book/365577/
#623 is by Tsukasa Jun. I know it doesn't look much like his style, but it is. I'm pretty sure it's just a stand alone pic and not attached to an artbook, or anything.