Flex Projector is an interesting tool that allows you to make a custom map projection by adjusting the location and shape of meridians and parallels.
Here's a nice projection I defined to maximise the area of tropical vacation spots 8^)
The downside of this app is that as far as I can tell, there's no way to export a projection once it's defined. Instead, you have to use the Flex Projector application to reproject datasets. So it doesn't look like you can make your own custom projection for use in say, PostGIS or OGR.
I guess it might be asking a little much for the app to spit out some custom Proj4 C code with an associated EPSG code and CRS WKT... But hey, it's an open source app, so come on someone - how about it?
On Pair Programming
5 days ago