AC3Filter - it is a DirectShow filter for AC3 decoding to play .AVI with AC3 audio tracks and MPEG2 (DVD). Distributed absolutely for free (FREEWARE, OpenSource).
Features:

Full AC3 standard compliance.
S/P DIF passthrough support.
Decode MPEG2 PES streams.
Multichannel output support.
LFE support.
Dynamic Range Compression (DRC) support.
Overflow control and automatic gain control.
Gain levels control.
Bitstream information (channel format, bitrate, sample rate, coupling freq and maximun freq).
Input/output levels.
Matrix mixer with direct control of mixing matrix.
Stereo to multichannel mixing.
DolbySurround/ProLogic/ProLogicII downmixing.
Delays for each channel.
Equalizer.