A year ago, I owned over a hundred domains, so I got rid of the ones that I wasn't likely ever to do anything with. I now have about eighty, and have content on perhaps 40-45 of those, but have plans for the others. There are some that I have just because I bought the .net and the .org along with the .com -- they just redirect to the .com site.
I have created sites that are devoted to very small North Dakota towns, most with populations of under a hundred. It's an easy niche and since these towns don't even have official municipal sites, I can gain the number one or two position shortly after Google discovers the site, usually competing only with the Wikipedia page, which will include a link to my site anyhow. It is a tight niche, so these sites don't see big numbers, as far as traffic goes but I have a site with more than 300 pages on a town with a population of under 50, so it is very likely that anyone who has ever lived in that town, or known anyone who has lived in that town, will come across it sooner or later. Much of my traffic comes from email messages, or from searches on the names of people who have lived there, since I include biographical sketches on early residents. The nice thing is that, although I spend a great deal of time building the site, it's a one-time job, other than to add some little bit that someone might send me, like photos or additional historical information, and I don't have to spend one dime on marketing.