Overview
lsof or “List Open Files” is a favorite in my free software toolbox. It is so versatile there are few things you can’t do without it.
Here are some examples of real world lsof usage and a few things things you might not know it is capable of doing.
What process is holding onto /var/log/messages? lsof is [...]

Eric Butler did a very good writeup of this years Southern California Linux Expo. Instead of writing a rehash, you can read his post.
One particular thing to note is his mention of Sun’s true commitment towards Linux.