Virtual Audio Cable - Ever wanted to record your speaker output? (loopback) This is for you.
Guitarix - Guitarix is a simple Linux Rock Guitar Amplifier for https://alternativeto.
Loopback by RogueAmoeba - Get all the power of a high-end studio mixing board, right inside your Mac!
Ampire - State Space Modeled guitar amp/cab/stompbox simulator.
JACK Audio Connection Kit - JACK Audio Connection Kit|Home
TH-U - World class guitar amps and effects for PC and Mac