Shift coverage for very small businesses: employee won't be in to work, sends SMS or email to site that then sends out a notification to all other employees to see if anyone wants to cover his shift. I've seen this for large chains, but nothing for smaller businesses and I know they have problems with people not showing up. My wife used to manage a bar and she thought it was a waste of time, but I think it can be sold.
Easy online scheduling for plumbers, etc. Prospect emails/requests a visit from site. Message sent to plumber's phone if he's in the area. He can refuse the appointment or call the customer with an ETA. Selling point: on the go scheduling without a receptionist.
Estimation software for carpet cleaners. Enter size, furniture, materials, etc. And it calculates time & materials. You need domain knowledge, but there is a ton of available material online about carpet cleaning. It is also one of the fastest growing and most profitable janitorial services.
Just a few off the top of my head.
Oh, the one I fooled around with and then abandoned:
Management software for small rental properties. Webapp aimed at people renting out rooms or single homes. Automate the process from applications through rent collection and discharge. Collecting rent, especially, can be a pain in the ass if you don't live near the property.
Have fun and best of luck. Hope this is a lively thread!! Contact me if you want more detail.