One of my essential tools on the road in the UK is the SkypePhone via 3. It lets me reach people quickly and keeps me in the cabs not on the tube. The money I save on International and Skype calls goes to the cabbies :-)
I was happy to see that Threee have extended how long you now can use your Top Up credit from one month to three months. Of course, you still need to pay for a data plane, but at 5 pounds a month, 2.50 pounds for a week or .50 pence a day, that's still a bargain.