Well, it had to be first! Writing articles about anything you're interested is completely free at Hubpages - and you can get paid for it through what is called a "revenue sharing" system with Adsense.
Put simply, that means that the more popular your writing is, the more you can earn because Hubpages share the advertising revenue with you. There's also an opportunity to earn further money with moe advertising revenue options.
The best thing you can do is try it for yourself and Join Hubpages. With Hubpages you simply write about things you know, things you hate, things you like. There's a whole section of questions that I find invaluable for writing hubs as I simply go to the questions section, find something I know the answer to, then press a button to create a hub about it - usually about 30 minutes later I've created the whole thing, including research and finding some nice photos to go with it!