Bontrager's ACX is standard issue on most Trek and Gary Fisher bikes, and even though the evenly spaced blocky tread was pretty slow, it'll grip in a much wider range of conditions than most other tyres.
You'll notice the extra grip in loose or damp/ muddy ground. Dual 'Gum-Bi' compound wears relatively fast, but it makes them impressively sticky on the dry rocky, rooty stuff. You'll be slipping a lot in real winter wallow holes, but it clears fast once you're on drier ground and it's a decent 'one tyre does it all' choice.
We really like the Tubeless Ready versions of Bontrager's tyres too. Basically they're a lightweight, supple walled carcass but with a tighter tubeless-ready bead on them. This means you can either run them with a tube without any weight penalty, or stick them on a sealed rim, fill with Bontrager's Super Juice sealant and go commando. Price is good for the versatility too.