The answer is probably a live round, but anything that will work inside the house and last longer than one episode of Hawaii Five O?

I use fired cases in the 22LR for snap caps.

http://www.cabelas.com/prod-1/0006919220742a.shtml

.22 snap caps; they are not all that great

I don't dry fire .22's much, but sometimes I'll put fired cases in the cylinder for practice.
After I smack 'em once I push them out with a nail or little screwdriver , spin them a bit to get a fresh spot on the rim, then have at it again.

They get a little tight after three or four hits, but I've got so many cases I'll never run out of them.

Tipton has several styles of snap caps. They have the flat front caps like those in the link above and also have round front caps which can double as "dummy rounds" to practice reloading. (mostly for revolvers and speedloaders)

Midway has a package of 10 for $3.99 and a package of 25 for $6.79. They work well for me.

Midway also has Pachmayr snap caps that will cost you $6.99 for a package of 24. I've never used them but Pachmayr has a good name so I'm sure they are fine.

Thanks for the replies on a not-very-exciting subject.