I've got a Charles Owen Pro II, is vented front and back and comes with a removable silk. It comes in black, silver and I think pink, its around £90-£100 but is much nicer looking than some vented hats - I thought my saddler was trying to sell me a cycle helmet when I saw the other ones on offer!
The Champion Ventair is a really nice hat at a reasonable price - I think mine was around £70 - £80 or so. Is very comfy - the padding on the inside actually stays nice and squishy, unlike other hats where it tends to go flat.
It also keeps my head ventilated very well - no more "hat hair"!