Next task on my list looks like a fun one, integrating payment into a custom e-comm site using Active Merchant.
Documentation seems severely lacking, which could be to the simple looking usage scheme. I might try to write some docs as I go, and if I do, I’ll post them somewhere or consider contributing them.
So far I managed to dig up how to install it – the easiest way seems to be as a plugin – from within your rails app:
ruby script/plugin install \
svn://vault.jadedpixel.com/active_merchant/trunk/active_merchant