Contribute does not mean debug thousands lines of code per day or write documentation, you can help in many different ways and spend quality time with some very interesting people !
I will talk to you about my own experience:

A few months ago (Oct 2011), the NetBeans community was looking for two new plugins validators. I had never joined such a project, and I decided to try to reach one. I took the time to reflect on my motivation (everybody has already started a great project, then just give up one week later : I wanted to avoid it) and has therefore responded to the announcement in a honest manner. I let you imagine my excitation (and pride !) when it learned me that I was chosen.

What is the validation of the NetBeans plugins ? It is very simple and explained on the webpage : http://wiki.netbeans.org/FaqPluginVerificationDecide.
To summarize, we are informed of requests for audits of plugins, and - after test - can choose if the plugin will be validated or not (Note : we are not required to validate all plugins). A plugin is validated if at least two testers validate it. If not even a validator invalidates a plugin, it will be considered invalid.

A plugin is validated if it can be installed on NetBeans, disabled, and its installation and uninstallation will not corrupt NetBeans. The criteria of qualifications to check are explained on the webpage : http://wiki.netbeans.org/PluginPortalQualityCriteria.
Validate a plugin can be fast and interesting. Interesting because it allows you to discover plenty of interesting features. In addition, the NetBeans Plugins Validators team is very cool !

That's all ? Yes, that's all.

Many free and / or Open Source projects seek help (NetBeans, Eclipse, Firefox, Open Office, etc.), and offer very nice works. Then be not frightened, and go ask them what kind of assistance they seek !