Oscillo is the simplest and minimal oscilloscope Mac app that focuses on use with synths.
You can discover synths in a traditional way with backup of the latest technology.
It spends significantly low cpu%, so you can keep watching it without a worry.
Select your desired audio input device and channel, you'll watch typically a few cycles
of the wave. Select note name to pitch to the note you play, then the wave will be locked
if the note is tuned well. Hit Macro button (or letter M) to watch the note entirely.
Hit Pause button (or space bar) to pause/resume Oscillo.
Mac OSX 10.6 Snow Leopard or later, both in 32-bit/64-bit modes.