I'd have to say my aquarium vacuum. It makes life 10x easier. Before I had it, I spent 3 hours doing a not so effective water change on one aquarium. Now it takes me maybe an hour to do all six (or seven?) of my aquariums.
Hah!!!.. & just this afternoon, I "manufactured" a second syphoning device, allowing me to empty the tanks twice as fast.... refilling, however still takes the same time, but I managed to water change 17 tanks today (90%)... This is progress!