It takes about 30 minutes to get started with google adsense. For the cost of some space on your website and some effort to get the code in you can for little effort earn ... well, possibly peanuts per year depending on the popularity of your site.
But I don't want grotty adverts on my site! I hear you cry. Quite right too. By default google makes its best effort to work out what your page is about and serve relevant ads, but you can also restrict ads by keyword. Google text-only ads are not very intrusive and you can specify an exact colour scheme to match your site.
If you want more control over what you advertise, a better plan might be joining Amazon affiliates, which allows you to advertise a specific product on Amazon's site. That, or go into business yourself.
Of course the fact that I mention these two big names is yet another example of the winner-takes-all nature of internet business: the bigger you are, the bigger you get.