With pwnpi I use it for network stuff, but my arch sd card I use for trying out custom config files. If they work out ok, I send them to my main machine to use.
When my new workspace is set up, I'll be getting more into electronics and arduino. I'll be using the Pi to code. i'll be making things like clocks for starters, moving on up to robots.
Let me get my fanboyism out of the way-the Raspberry Pi is the best thing ever and there is nothing that comes close to it! (I know that is a false statement). Anyway. I have a Raspberry Pi and I use it to learn to program, learn the terminal, and just amaze my self with how great it is at a low price. I've seen web servers, clusters, and them used just to learn programming. Personally, I will use mine to learn how to program, networking, and just how computers work in general. I will be making a cluster(like a supercomputer without the super). The coolest things I have seen done is a cluster and a garage opener(thats one of those little things in life). Im off my soap box now.
One of my mates uses it to try and program his own Linux distro or edit other distros. He has great fun doing it and has produced some great results. You dont have to worry too much about messing things up either.