I'm interested in selling http://www.seasonalysis.com. The site has been around for over 4 years and has a wealth of seasonal market data. We have subscriptions ranging from $50/month to $5000/year with several at the top membership tier. I think with the right marketing campaign and site content it could be quite lucrative but I just don't have the time.
Seasonal stock market tendencies are real. I created a website (Seasonalysis.com) which quantifies these tendencies. Past performance is no guarantee of future returns but the information gleaned can be incredibly useful if combined with other indicators/analysis.