stefkeB

You can try any of the following free plugins in your regular VST host, to emulate guitar amplifiers or FX: 

• SimulAnalog Guitar suite
• TubeBaby
• Jesusonic
• Fretted Synth

These are all VST's freeware (not Open Source) for Windows. The commercial Studio Devil has a free British Valve Custom (BVC) available for PC & Mac. The commercial Amplitube is obviously not free, but there is (was?) a free Amplitube UNO application, which is a standalone amplifier simulation. You might be able to hook it into your other software using audio routing (e.g. SoundFlower on OSX).