Any site that turns revenue to pay for the cost of the domain & a years web hosting is successful, in my opinion. I'm a strong believer that anyone can turn revenue on any topic of site as long they take the right steps.
Before you dive into the next site think of things you already know. You mentioned Linux, is that an O/S you know well? The majority of of my sites are based on topics that I know or at least have easy access to resources. Some of these I wouldn't believe would turn profit or was too competitive but yet, I have them turning revenue and sitting high in the search engines.
Take Linux for an example. The majority people searching about linux wont use the single word "linux". They are most likely to search for things like linux help guides, software for linux, linux how to guides etc etc. Run searches on different linux key phrases and look at the results number, you'll see a big difference.
Although Linux is open source sites still need to advertise and there is always revenue to be made.
You don't need to be a professional at anything, just create a site based around what you already know, even if its baking cookies at Christmas.
