An idea that has interested me since I was very young are "everyday arbitrage" opportunities. Even before I knew that FI was possible or had heard of MMM, I was always a fairly prolific saver, with an eye towards opportunity. In middle school and high school, I would buy things in one area and "flip" them in another (e.g. buying something at a garage sale that I knew I could flip on eBay for more). So I'd be interested to know what opportunities the MMM community has come across and how they participate.
For example....
When I was growing up was when I played online poker with my friends and was the only one with a bank account I could link to the sites. Because my friends all really wanted to play, they would give me $20 in cash for $15 in online poker money, and I'd pocket the difference.
Another example was when I would buy new iPhones when they first came out, and resell them on Craigslist to tourists in NYC who came from countries where the iPhone wasn't yet released. I was able to pocket a quick $200 each time.
Would love to hear what little niches others have found, and think it would be an interesting topic to explore. I also may make a blog post about it in the future!
http://www.valueiswhatyouget.com