There’s a website called Tinkersynth, where you fiddle with unfathomable controls to make colours and patterns on a little screen-within-a-screen. It looks like this:
When you see something you like, just click the “Download” button and a png file arrives on your computer.
Gather together a handful of those png files, then head over to moo.com and use their custom greetings card design-your-own feature. Hey presto! Nice little cards made out of bits of internet.