I could go on for days...
1) Free
2) Highly customizable
3) Thousands of programs comparable and often times compatible with Propietary software (i.e. OpenOffice).
4) Viruses are pratically non-existent
5) Stable (no blue screens of death)
6) Lower hardware requirements
7) Relating to #6, it runs incredibly fast.
8) Hundreds of different Linux Versions (called Distros) that are tailored to different needs.
9) Open Source = Faster fixes for bugs in software
10) Many distros can be booted "live" which means you can try the OS without installing.
Oh did I mention, it's FREE. I personally use Ubuntu, but for a brand new linux user, I would recommend Mint.