My first experiences with open source software:
a) Checkstyle - a code convention checker for Java.
The setting up of the command line tool is a bit non-user friendly but probably this is the case with any non-proprietary tool ;-). One major observation is the number of plugins written by open-source enthusiasts to integrate the same into any IDE (Idea or JBuilder) which makes the setup a one-time activity. Coming to the real power of Checkstyle, it is its customization to write new rules. (For example, adding a rule to check the number of methods in a class). So it gives the ultimate freedom if somebody wants to get more power from the basic framework which is where time should be spent for delivering more customized benefits.

b) OpenOffice - Office suite
Breathtaking is the word. It is just amazing how an open source project can deliver just a good product. You have to see it to believe it. (come to think of it that i haven't even had a deeper look into many of its features). . The features are more less comparable to Microsoft's and the UI is also good. It has few features like saving to pdf which is a real value addition. It can open any known, popular format (staroffice, MSoffice) and save them too. Also, it can save the same in xml formats which can deliver real value to some applications (which can use these formats to read, write or do whatever) and one doesn't have to bother about licensing.