It used to be that a portable computer was an expensive luxury. That's all changed with the rise of a new class of computer: the Netbook.
Netbooks are usually defined as small, low-power, ultra-portable computers with a small price tag. These are mostly fully featured, but not as powerful as full desktops or notebook computers.
These Netbooks are not well suited for games and big programs, but perfect for email and surfing the web. That's not a big problem these days since so many things run as web applications.
There are some excellent values out there - In order to protect my beloved $1200 MacBook, I picked up a new Netbook for work. It has a 10" screen, 1GB of RAM, and 160GB hard drive - for only $350. This computer did something pretty amazing: It impressed me!
With wifi connections available everywhere, there is no need to be tethered to your old desktop at home. Grab yourself a small, light Netbook and hit the road! Now you can answer emails or write that book wherever you may be.