I have a simple IP lookup site (http://showip.net) that I created many years ago. I created it because I needed it (but mainly too learn at that point ;) Since then it has gotten better and better indexing on Google until it reached a steady level at around 100.000 - 130.000 visitors / month.

The ads has been fluctuating quite a bit, but overall it has been going steadily up. Currently I earn somewhere between $250 - $500 / month.

I haven't done much but a bit of tweaking now and then since the initial version, though I have had plenty of plans... I have changed the GEO IP location DB a couple of times, but the only major change was a rewrite from PHP/MySQL to Node.js and a GEO IP database directly in NginX.