Ignite Amps has released NadIR, a free impulse response convolver for guitar and bass cabinet simulation.
NadIR is designed to perform pristine quality convolution in real time, while being light on the CPU and easy to use, providing advanced built-in filters and delay controls to let guitarists and bassists shape their tone with ease, without the need to be professional audio engineers.
Price: Free. It's available for Windows and Mac OS X in VST and AU plug-in formats.