io.js is a spork (hoping to merge back into main-line) of Node.js (or "io.js is an npm compatible platform originally based on node.js™). It can be downloaded and installed from their website. I'm currently running v1.0.1.

It updates node to ES6 and a recent V8, and has done a fantastic job! Unfortunately it removes the node binary, not everything on npm is compatible, and very little on npm has been tested for io.js compatibility . This meant I wanted to run node and iojs side-by-side.

To install itself, iojs symlinks itself to the path of the node binary.

$ ll /usr/local/bin/{node,iojs}
16864 -rwxrwxr-x. 1 root root 17268324 Jan 14 08:29 /usr/local/bin/iojs
    0 lrwxrwxrwx. 1 root root        4 Jan 14 08:29 /usr/local/bin/node -> iojs

This symlink should be removed if you want to run Node.js and io.js side-by-side, or just revert to node.

(n.b. all these commands need be run with sudo as they're owned by root)

If you don't want io.js on your system and want to revert to node, remove io.js first:

$ cd /opt/io.js # or where it was cloned/downloaded 
$ make uninstall 

Or to keep the iojs binary, just remove the symlink it writes over the node binary with:

$ rm /usr/local/bin/node

Because node was overwritten with that symlink, it should be re-installed using your preferred installer. To install nave (my preference):

$ curl -o /usr/bin/nave && chmod a+x /usr/bin/nave; 

Then install node:

$ nave usemain stable
installed from binary
$ which node

Which, on Wednesday 14th January 2015, is;

$ node --version