The Crowne Plaza Changi (landside at terminal 3, but easy access to other terminals via the train) has a day rate for 8 hours from 11am IIRC. I've used it a couple of times coming back from India, and loved it.
It's definitely not the cheapest option (you could probably get a whole night at a nice hotel in town for the same price), but it's a damn nice hotel (good food too), and you sure can't beat the location for convenience. There's even a good spot for kaya toast for breakfast just outside, if you arrive before 11 (as I suspect you will if you're on the Jet flight I'm thinking of).
If you haven't seen the sights of Singapore before, I'd definitely say getting into town is a better use of time - but if you just want somewhere to crash out and do some work for a few hours, the CP is great.
Cheers,
Danny