In the UK, I use http://www.freeserve.com/ (free).

For the rest of the world, I use http://www.attbusiness.net (the former IBMnet, which as noted earlier in the thread charges a rather stiff 15c/min "roaming" fee outside your home area), and http://kqtravel.com/ (the former Eunet, which charges about 5c/min in most places, and provides access to the IPASS POPs as well as its own). Both have worked well for me in a dozen European countries, Japan, China, and Australia.

There's also http://www.PrestoAccess.com/ (10c a minute toll free in Europe). I've not tried it.