When you add music to presentations or videos, you must be very careful that you are not breaking copyright - you are probably breaking copyright if you use music from your personal collection of CDs.
One of the places I recommend to find free music for download is Jamendo. This is a website where musicians leave their music for others to download and share under Creative Commons licence. You can find music in most genres - jazz, classical, easy listening, electronic, dance, hip-hop.
Have a look at this post by Richard Byrne for more information, and other places to look for free music and sound effects.
Is there any web site you'd recommend to find free music for presentations etc?