I use the Winamp media player all the time. I don't trust the security of Windows Media Player and iTunes is bloated. I just need something that will play the MP3s for background noise while I'm working.
Good stuff: Winamp is the number one Free alternative to Windows Media Player and any other pay-to-use player. It is Free.
Not so good stuff: Winamp tries to do everything, which means it is not as customizable and user friendly as it advertises. And, many consider it bloatware, meaning just what it sounds like - takes up a lot of space on your HDD.
Opinion: if you have lots of space on your HDD and you want a Free music program that has lots of information (just using Google, Yahoo! etc.), then this just might be the program for you.