du -sh /*

rm -rf /tmp/*
rm -rf /var/tmp/*

Uses aptitude package manager.

Paste this ant the CLI then run rmkernel to remove all kernels except your running kernel.

rmkernel () {
        local cur_kernel=$(uname -r|sed 's/-*[a-z]//g'|sed 's/-386//g')
        local kernel_pkg="linux-(image|headers|ubuntu-modules|restricted-modules)"
        local meta_pkg="${kernel_pkg}-(generic|i386|server|common|rt|xen|ec2)"
        sudo aptitude purge $(dpkg -l | egrep $kernel_pkg | egrep -v "${cur_kernel}|${meta_pkg}" | awk '{print $2}')
}

http://www.omegaprojex.com/index.php/2008/11/10/moving-shares-and-users-home-folders-to-a-new-server/

http://alloraconsulting.com/it-solutions/76-windows-server-2008-low-space

http://serverfault.com/questions/8187/whats-the-best-way-to-move-c-users-to-d-users-under-vista-w7

For Windows 7 or Server 2008 enter this command:

rd /s c:\$Recycle.Bin

For Windows XP, Vista, or Server 2003 enter this command:

rd /s c:\recycler

TreeSize Free is a very useful tool for finding the problem:

http://www.jam-software.com/treesize_free/

This is a short script that is probably best used as an emergency 'free space now' tool:

@echo off
rem Script to clean up disk space on SBS 2008 servers
rem Downloaded from SBSfaq.com
rem V1.0 - March 28th, 2010
rem Certificate Services Logs
net stop "Active Directory Certificate Services"
del c:\windows\system32\certlog\*.log
del c:\windows\system32\certlog\*.chk
del c:\windows\system32\certlof\*.jrs
net start "Active Directory Certificate Services"
rem IIS Log Files
Del C:\inetpub\logs\LogFiles\*.log /f /s

CleanUp may be useful to clean up Windows Profiles:

https://bitbucket.org/dev247/cleanup/overview

If you are building new systems:

http://support.microsoft.com/kb/949977