I believe every system administrator is used to open source by now. From Apache to Firefox or Linux, everyone uses it at least a little bit.
However, most open source developers are not good in marketing, so I know that there are hundreds of very good tools out there that very few people know.
To fill this gap, share your favorite open source tool that you use in your day-to-day work.
*I will post mine in the comments.
Best Answer
I love PuTTY !
The PuTTY executables and source code are distributed under the MIT licence, which is similar in effect to the BSD licence. (This licence is Open Source certified and complies with the Debian Free Software Guidelines.)