Google AdSense used to work brilliantly for me and my forums, and enabled us to earn a small but significant income that we could use to invest in extra features, advertising, graphics & such for the site, but this was many years ago. I've heard many stories of people not having such a good time with AdSense in the last five years or so especially, so I'm unsure if there's another network that people who are running ads on their website are generally using now, or if people are just putting up with a mediocre service from Google.