According to pcworld, Open Source software is good for business for exactly ten reasons. :-)
Summarizing the points we have...
The more people can have a look at the source code the more likely is that a bug can be found.
"In general, open source software gets closest to what users want because those users can have a hand in making it so."
If you don't like the way something works just dig with your hands into the code and change it by yourself.
You are not dependent by the vendor's priorities, timetable etc. Similar to the previous point, if you want the software to develop in one direction, which is not in line with the vendor, you can do it yourself or paying someone for it. The point is, you are allowed to do it. On the other hand you cannot modify proprietary software.
You (and just you) decide WHEN and IF is the case of upgrading the software.
Open Source software often adheres to Open Source standards. This makes the exchange of files/data among different application easier.
Of course being Open source the application is transparent. Anybody can see what it does behind the scenes.
The software can be supported in different ways. There are communities of people/developers ready to help (free of charge) as well as private companies.
Obviously you can get Open Source software for free.
Try Before You Buy
Being free, you can always try the software for as long as you wish. No 30/60-day trial period...
The whole article can be found at PCWorld.