Some possible ideas for Cross Promotion…
Indie Games Cross Promotion
having a recommended indie games section on your website.
basically a curated page of indie games that you recommend…
could also be easily done by simply setting up a little tumblr on a subdomain.
ShowMeTheGames.com (by Cliffski)
Indie Games Sale
… running a special sale with other devs
The Dirty Dozen Discount (organized by Cliffski)
Indie Cameo Cross Promotion
guest appearances of other indie game characters (of course permission is needed beforehand)