I would go to Elance and look through the things people are paying for. If you can find something that lots of people are buying but can be commoditized you've

I would go to Elance and look through the things people are paying for. If you can find something that lots of people are buying but can be commoditized you've found a winner. Since you are experienced in PHP, make some PHP component that people want but the current solutions are crap. Web Developers will easily pay $100 for something of quality that will save time. I think it is easier to sell a digital product than SaaS if you want to do this part time. It's nice to get your money and have no further obligation to your customer. As far as marketing, you can submit a PAD of your software product. That will give you a surprising amount of free traffic. Also, you can giveaway licenses of your product via bloggers. To accept payments I use FastSpring. It looks professional and there are no setup or monthly costs (so no risk). Good luck!