Skype is well known for its service bringing people across the world to talk freely with each other just using their computer and high speed internet connection. For the most part, it is a free service and Skype has beeen a very popular voip application for the last few years among the 'Computer generations'. They also have prepaid service, SkypeOut, for user to call non-skype users for few cents per min.
However, you still need to download and install Skype program to actually use it, which mean, you have to bring your personal laptop anywhere you go. Worry not! There's a new kid on the block to solve your dilemma. It's called Gizmocall.
Gizmocall.com provides web-based voip service. All you need is to have a) a decent headphone, b) a decent microphone, and c) any computer connected to high speed internet. In fact, if you are in your friend's dorm room and need to call your mom who lives hundreds of miles away, you can just sign on gizmocall.com and start talking.
Like Skype, it also offers prepaid blocks credits for calling non gizmocall users. The only downside is, gizmocall is strictly for making dirt cheap phone calls. No IMs, no video messaging/conferencing.