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.

What's New
AC3 decoding bug fixed (Issue 1)
Channel order bug fixed (issue 43)
Remote control using messages
Write version number to the registry on install (issue 17)
Command line interface for AC3Config utility (run ac3config /? for help)
Swedish translation added (thanks to Niclas Burgren)
French language updated (thanks to Philippe AGUESSE)