The cheapest/best value for a standalone MiFi device is 3 (£60 for the MiFi), with some pretty good PAYG deals. It's even better value if you grab your self an iPad sim only contract and use it in the MiFi instead (£15 a month for 10GB).
Other than that, the iPhone got that ability with iOS 4.3. 3 enables it as part of your contract. Vodafone, O2, and Orange all charge extra for it. Tesco Mobile won't give you it at all.
There are also Android devices like the HTC Desire that do it.