Quick and easy installation

Max OS X

Install homebrew then:

brew install node.js  

or get your mac up to speed with cider.

Ubuntu 9.04

sudo apt-get install build-essential libc6-dev-i386 lib32stdc++6 git-core
sudo ln -s /usr/lib32/libstdc++.so.6 /usr/lib32/libstdc++.so
git clone git://github.com/ry/node.git
cd node
./configure 
make
sudo make install  

thanks Travis Swicegood.

Install from packages

RPM and DEB packages for Node.js.