Here you go... I got it for like $16 at Target
http://www.masterdoorlock.com/nightwatch/index.shtml
Other brands might have similar products too. It is incredibly simple... when you pull the latch out, the cylinder its attached to slides into some grooves that prevent it from turning, hence also keeping the tumbler from turning. When I get home tonight, I can take some pictures of how the internals work and post them.
I purchased the lock to put on the door to the garage. Since it would only be locked at night, I didn't worry about getting something that could be pinned to match all the others locks. However the latch mechanism fits on our existing deadbolts (Dexter brand), so I was going to just replace that part so I don't have to replace keys.