I took my rear wheel off, put it back on, no biggie. Then I roll my bike forward and saw this peice laying right under the tire. Its gotta go somewhere! It was on the side my brake disc is on. I thought it was some sort of retainer for the pads, but I cannot find anywhere this peice will fit into. Anyone help me out here?

 

It looks like an "anti rattle springy thingy" that keep your brake pads from rattling. It lays flat in the caliper, just like it was laying flat on the floor when you found it. You can run the bike without it. I took mine out of my front calipers on my F2 track bike. No problems.

 

It looks like item #6 here.

Honda Motorcycle Parts 2006 CBR600F4 A REAR BRAKE CALIPER Diagram

 

their both right. i just put a new rear tire one mine. mind you it doesn't go on the caliper but yet the back end where the pads slide.