Hard to say because I rarely buy or build anything that I wouldn't deem indispensable in my gear collection. Two things come to my mind and are crucial in my work process though: my octaver beefs up any thin signal and adds thickness to my sound sources, and a homemade compressor makes them all shine. They're both discrete pieces of gear, they don't hide my lack of creativity or my mistakes, they don't change my sound at all. However, they really make everything everything nicer (I don't use the octaver on everything but I use my compressor with everything: synths, drum machine, bass, guitar, etc...)