[eluser]David U[/eluser]
Lots of people use Microsoft products and pay a lot for them, despite the fact there are free equivalents for almost all of them. So there are still lots of illogical people out there if you want to make money.
You'd have to work hard to do better than Bamboo Invoice, though, since it's written by one of the gurus behind CI. (Derek Allard)
As a small businessperson, what I'd like to see is a simple accounts package - going a bit further than invoicing, so it can include payments out, record all transactions on, and reconcile, two or three company bank accounts, credit card accounts, etc., do end of year statements and quarterly VAT statements (VAT is a tax we have in Europe) and a few other boring accountant things. None of it is rocket science, it's just that you fall asleep before your accountant finishes explaining it to you.
The current packages (Sage, Intuit, etc) are all expensive and clunky. (eg Intuit insists on using IE as its interface.) A free or cheap open-source package, without unwanted bells and whistles, would be really worth having, and not too difficult to design.
Derek already has the basics in place and if he ever got round to tweaking BI to do these things, there's a HUGE market.... and if he doesn't, maybe you could?