What is this bracket?
 

I did something dumb today on my new bike when starting from a stop, which caused my chain to fall off. No biggie. When I got down to really fiddle with it, I found this bracket loose just above my front crank. It doesn't seem to serve any purpose... anyone tell me what it is? It looks like just a large, plastic hook.

Is it just a cable guard of some kind?

I guess you must have bought the bike used. What you have there is a chain watcher (though not necessarily that brand). It's supposed to be mounted just above the inner chainring and positioned so if the chain wanted to fall off, it would catch it and guide it back onto the sprocket.

The irony is that if it were in the right place, it would have done it's job and you'd never known it was there.

It's a "chain watcher", and installed correctly, it's there to keep the chain from falling off. The one you show is rotated way too far to do any good. Reposition it so the plastic "finger" just clears the inside of the chain when in the small chainring and tighten the clamp to hold it in that position.

Ah, thanks. Maybe it was installed at one time, and it just came loose and turned into the place where I found it. Thanks. :)

Make it so it almost, but not quite, is touching your chain, in the small chainring..