Ask HN: What program do you use to test your skills

I've been teaching myself Go for the past week and last night I thought I'm at a level to code something actually useful to see how good I've become.

I ended up writing a port-scanner. (that only checks for one port at a time but changing it to scan a range of ports is pretty trivial)

Now I want to know what your go-to program is when you want to test your skills?