What crazy world do you live in. You will never ever have more free time than you did in college. NEVER! Ignore this statement if you are working and going to school
A DM screen. I bought it because my roomate and quasi-roomate pressured me into DMing for them over Spring Break, and I haven't gotten them together for a period of time since.
Since I DM online, the DM screen isn't all that usefull to me
I just got the Hackmaster GM's Shield yesterday and I know for a fact that I'll never run a Hackmaster game.
The shield is still sweet, though. A masterwork of engineering. Sometimes I wish I had a similar shield for my other rpgs, but just as often I'm happy that my other games don't have that amount of tables.
Oh yeah... and I'd say (to answer the question) probably Encyclopedia Magica, all four volumes, which I've used probably four times since I got em years ago. Oh well, it's a resource. I've also hardly ever used anything out of Relics and Rituals.