I don't really consider the project itself to be objectively cool (it lets you receive files that are too big to send over email directly to your Dropbox folder - you can send files up to 1 GB to me at http://dbinbox.com/cgenco), but it's really cool to me personally because:

* it's the first project I implemented payments through Stripe with (which is an amazingly cool API)

* it's the first side project that's generated enough revenue to quit my day job

* it gives me an active excuse to make other, cooler things that will eventually spin off as their own projects (like a cheaper Mailchimp integrated with Rails on SES)

* it's the first time I've ever needed to scale beyond one server, so I get to learn all about multi-server deployments and keeping your data (on RDS) separate from your app servers separate from your worker servers

* I'm getting really good at chugging through support emails, and thereby email in general. I've got a really nice inbox 0 flow going on that I plan to write about soon

I also now get to work whenever I want, take as much time off as I want, and not overanalyze small purchases because it's likely I'll get a notification from Stripe that I've just received an order of magnitude more money than I'm deliberating over.

It's a blast :D