When I'm not making my own shelter I have a Eureka Solitaire that I use. I've had it for a long time and I like it. It's small and light weight, but still reasonably durable. They're also pretty inexpensive and readily available. My only problems with it are it's a bit tighter than many more modern designs, and it's not really a winter tent (as the top is open mesh with a fly cover and there is no center support). I've never had a problem with it though.

Hmm, I think it's about time to get myself a new winter sleeping bag and tent, maybe a nice bivy bag too for the summer.