Things. [0]
It's based around the Getting Things Done (GTD) methodology [1]. I love it. Best $10 I've ever spent.
The company also has a habit of charging $TEXAS for upgrades for new major releases. That is totally within their right and they deserve all the cash they can get -- but that reality may affect your buying decisions.
I did the Things thing for a while but determined the money leaving my wallet wasn't worth it for me. That said, it's among the best, cleanest, highest quality Mac software I've used.
I'm still waiting for someone to correctly use a threeve, though.