I just found out about Jajah after a TC article. They seem to have some interesting tools in terms of VOIP services. Worth a look.

JAJAH.Direct
From any phone, you simply dial a unique local JAJAH.Direct number for each of your international contacts. Save these local numbers in your phone then call the world like it’s local.
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JAJAH.Web
From the web, you can initiate a call with no headset, no download, no software installation required. Use any phone, start saving now!
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JAJAH.Mobile Web
From your web-enabled mobile you can point your browser to mobile.jajah.com. …

Truphone recently launched Local Anywhere. I think it’s pretty cool. It allows you to make calls at local rates by purchasing their international sim card, so it’ll be very convenient for frequent travellers. Not having to manage various SIMs and unlocking and searching and topping up would be great.