Every sysadmin I know is a little different.  Some prefer GUI tools and others prefer the good old command line.  All of them though have a certain set of tools that they use every day.  Jon Buys at ostatic.com lists several of my favorites like free and rsync.  He offers great insight into the tools and hopefully will help you be a better sysadmin.(Assuming that is what you want to do.)  Check it out and then let us know what your favorite sysadmin style tool is…