If you've seen other good ones I'd be glad to add 'em to the list, this is just what I've personally seen.
Regard by Queasy. It's um ... Riff looks at Cain and Cain looks back but they never really catch the other looking. Also, Cain is jealous.
Night Under Cover by
chibirisuchan. Riff and Cain spend a night in a country inn and find something to do to pass the time. It is hilarious, you need to read it.
Master and Servant by
chibirisuchan. The only problem is that she doesn't think she's going to finish it on account of the conflict between the need to be in line with what would happen in canon and the nearly overwhelming desire to kick Cain headfirst into a magical funland of PUPPIES and KITTENS and GAY.
Where Love Is by
athousandwinds. Cain is a loving brother and a real nasty son of a bitch.
See That Boy by
athousandwinds. A boy sits in a cage.
The Devil's Weed by
athousandwinds. Cain is a boy who loves flowers. *coff*
Man of Words by chibiduo. The writing isn't as polished as I generally like in a fic, but it's a good solid story about Judith and Cain.
Peaches by
puppe. I thought it was cute, ok.
Five by
puppe. Five things that never happened to Riff.
San Souci by
dew_chan Things really could have been worse. Cain could have had a normal life.
Also Sakkit drew
Cain invading Riff's personal space as usual so muhahaha.