There are many open source projects that have hit the big time such as FireFox, Apache and of course Linux. However for each of the big names there's hundreds of smaller projects out there delivering quility applications that you could possibly make use of.

So how do you find these projects? Well take a look at http://www.osalt.com. This site allows you to search based on the name of a comercial product and will list the open source alternatives.
 
Image credit: Open Source Initiative (c)