Retron is a free mono tube amp plugin designed to add light to heavy tonal colouring & light/medium distortion to your audio. Retron is designed to give that old sound to anything it processes, and features effects including a phaser, delay with analog filter, fuzz module and a built-in nonadjustable compressor. The main filtering in Retron is an 8 stage lowpass filter, and there’s also a dedicated filter for the delay/echo effect with a choice of formpass, phatpass and lowpass.
RollAmp is a free guitar amp simulator for Windows and Mac.
Cypress TT-15 is a free guitar amp head emulation meticulously modeled after a very versatile, yet uniquely sounding low wattage amplifier.
Ace is a faithful free digital emulation of a legendary 1950s, American, all tube, tweed, 5W amp. They have added to the model two of the most common amplifier mods: adjustment of the amount of feedback and addition of grid stopper resistors, both of which can be turned off for the original amp sound.