This Coleman Fleece Sleeping Bag is pretty nice. It packs small, it's light and durable, and it's very inexpensive.

I've used mine as both a liner for my 32º bag in cold weather and by itself in warm weather.

The biggest downside to this bag is that debris sticks to it, you can't just lay it on the ground and sleep in it, or even just lay it on the ground. It's not hard to shake it out and brush it off though and it's machine washable.