Anywhere any time... I suppose wi-fi is out of the question then, where you HAVE to be near a 'hotspot' to gain access? By far the cheapest and also the fastest but most restricting as you cannot do it in the middle of nowhere... or anywhere outside airports and Starbucks for that matter
Alternatives are :
Mobile phone / GPRS / bluetooth - use your phone as a modem and connect to the net via GPRS. Fine for checking emails but slow and expensive for browsing.
Vodaphone / Orange 3G data card - connect to the net at near broadband speeds, great for email and for browsing. You will, however, need to be on a monthly contract just like a phone and the cost will soon mount up if you browse often.