Hm...
That's an interesting question.
If you are looking for a _product_, Forum softwares are extremely popular! Can you imagine how much money vBulletin ppl are making?
Other areas:
1. Content Management System (pMachine)
2. HelpDesk (perlDesk, though it's in Perl, it sells quite well)
I think these three products are in demand and probably (You never know!) will continue in demand.
I am myself in search to identify a web based app which I can make and then sell it to thousands of people! But I have not been able to decide. Trouble being, the above mentioned apps will really take lot of effort and I think you should write them in team of at least 2 people to make it really good!
All the best Justin!
Regards,
JD
P.S.: I have been fan of you and your script (ezSQL) the day I came to know about it! Thanks for that great script!