Window cleaning is good for a number of reasons: most able-bodied people can do it easily with little cash outlay, no skills, and reasonable good weather. It has the downside that it is hard work (manual labor) and not that easily duplicatable or offering residual income.
There are many great business ideas that offer both immediate revenue AND ongoing residual income: writing a book or a song, for example, selling consumable goods with repeat orders, owning a bank, is another.
Personally, I think I've hit the jackpot (and wish I'd thought up the idea!) with Merchants Barter Exchange: if you can talk to business owners, can teach others to do what you do, and are ok at managing people, it's a very simple way to make well over $200,000 a year and make great residual, ongoing money year after year. It also helps that I absolutely love what I do, so it really isn't work for me!
Looking forward to reading other people's ideas on this one.