About a year ago I built a number of micro niche sites aimed specifically at generating adwords income for high ppc terms. They weren't especially pretty but they were a good exercise and more than paid for themselves.

At the point I sold them (22 sites) they were generating about $190 a month in adsense revenue, all this from me buying 'keyword in domain' urls, having a relatively good eye for writing seo copy and building some inbound links.

I sold the lot on flippa for just over $1k, so I suppose it made me about ~$300/mo for a year. I know of some individuals who have over 1,000 domains running (across some of the most random niches you would just laugh at!) so I think it's a pretty decent passive business for them.