Free Truphone "Local Anywhere" SIM card with 5 pound sterling/ $7.50US credit from this site:

http://www.truphone.com/local-anywhere/freesim/

Use the offer code: z7un88

You can also try these offer codes, though they may be expired: ezy482 F23T5J

NOTE: It looks like this offer ends on March 19, 2010, so get your emails in ! Also, it took about 2 weeks for mine to arrive, so don't count on it for a trip sooner than that.

This will give you a local number in many places throughout the world, but with quite reasonable rates in the UK and USA. The credit is good for about 50 minutes calling in the US, you can check other countries. Nice for quickly switching SIM card into your unlocked GSM phone.

Truphone Local Anywhere works on nearly all unlocked mobile phones (an unlocked mobile phone allows you to switch between SIMs of different network operators).
If you are not sure whether your phone is unlocked, borrow a SIM from a friend who is on a different mobile network, insert it into your phone, and see if it works. If it works, that means your phone is unlocked and will be fine to use a Truphone Local Anywhere SIM.

Nice for transiting travelers, or those who want a relatively low cost local number in a number of countries.

Good Luck!

Doc