Elfen Lied had a manga but it was Based of the tv show not the other way around[like it usualy is]

no the realy is not that many independent source oav or movies out there
Most are based on Novels,manga,games or Products
sure there is gundam and some random other robot shows that make it on their own but they have a their special..^thing^robots^.. that bring them together

of corz there are some hentaigames that try a little extra
tsukihime is one of them its not very hentai to begin with but it is a hentai game so sure