There are many free amplifier simulators out there and some are as good, if not better, than their commercial competitors.
But there are so many of them! Which one should I get?
If you had to pick only a few ones, I would recommend:
- Ignite Amps' Emissary, very versatile,
- Mercuriall's JCM800 or Harlequin for a warm, vintage all-tube sound,
- Lepou's Lecto / Legion / LE456 for Metal,
- At last, for bass, SHB-1 from Ignite Amps.
In any case, the essential element will be the cabinet (the impulse response, or IR) that you will add after the amp sim. Depending on the IR, the sound you'll get will be quite different. The choice of the IR is really important to get the sound you're looking for. (details here)