I loved the Dark Ages outtake Donald described: Play the top 4 Ruins, or +$3 if the Ruins supply pile is empty. So much so that I implemented it in my Dominion A.I. and tried it out. In real life, I imagine the tedium of manipulating the Ruins pile every play was prohibitive, but I don't know.
Anyway, I like the idea of using Ruinses this way. They're all single units of functionality that can be easily tacked onto things and mixed and matched.