1) CellSafe - fits in your bottle cage: holds cell (of most sizes), keys, money. And it floats!

2) Tennis ball can (saw this one in a cycling mag). Same as CellSafe, but cheaper but not sure about the floatation thing.

3) Amphipod - this is what I use, cause I use both my cages for bottles. It's a little neoprene pouch (about 3" x 4") with a sealable insert inside. It has a velcro closure and a clip to hook to your clothes. I clip it inside my middle jersey pocket. I keep an emergency contact info card in the insert (so it doesn't smudge)and my cell. The 'pod will suffice only for the little flippy cells, not the longer open faced ones. I think I picked it up at a runners store.