This topic used to be known as Rooting your Pre but that term is inaccurate.
The Palm pre is a Linux based device. As it is shipped, the linux is a fully installed and operational copy, and communicates with the outside world using the Novacomm protocals (the same as used by the iPhone.)
Therefore, enabling access to the Pre was remarkably simple. It merely requires opening a port in the firewall to allow a user to telnet in and operate the underlying Linux.
The community quickly made available the OptWare library of over 1000 Linux programs, compiled for the Pre.
The community standard Pre with Linux access has installed the OptWare package installer, an SSH program (dropbear) and an SFTP program to move files on and off of the Pre.
Once this is done, basically, anything you can do on a Linux box, you can do on the Pre. The community is rapidly working on developing methods for linux programs to interact with the end-user shell known as Luna or Mojo in addition to access via terminal programs.
The following links will walk you through the process of obtaining access to the Linux system from a terminal program, and installing the community standard software.
There are three paths: