New version of AVS Proxy GUI (2010-10-01)
Here is a new version of my GUI for the Avidemux Avisynth Proxy:
Download including sources:
http://www.ziddu.com/download/13097852/avsproxy_gui1_2010-10-01.zip.html
Install instructions:
Please make sure that Avisynth 2.5 is properly installed on your system, before you try to use AVS Proxy. That is very important! Then simply put the "avsproxy_gui.exe" into your Avidemux install folder - that's where "avidemux2.exe" and "avidemux2_cli.exe" files are located. The path depends on the Avidemux install location you have chosen! Usually Avidemux is located at "C:\Program Files\Avidemux 2.5", but it may be different on your individual computer.
In order to enable the full power of Avisynth, a few Plugins are required: So please make sure that FFMS2.dll (FFmpegSource2, Beta-10 or later), AVSS.dll (DirectShowSource2), DGDecode.dll (DGIndex), DGAVCDecode.dll (DGAVCIndex) and NicAudio.dll all are located in your Avisynth "Plugins" folder (not Avidemux "Plugins" folder !!!). Usually the Avisynth plugins folder is located at "C:\Program Files\Avisynth\Plugins", but it depends on the Avisynth install location you have chosen. If you don't install all the recommended Plugin DLL's, then the AVS Proxy GUI will still run, but certain features (i.e. source filters) will not work and throw an error message instead! So I highly recommend to install all of the Plugins mentioned...
One remark about Haali Media Splitter and obtaining the DirectShowSource2 plugin: You need to install the "Haali Media Splitter" first. Then you can find the required file AVSS.dll in the "Haali" install directory. So you can simply copy it over to the Avisynth Plugins folder. More experienced users may also use 7-Zip (or Universal Extractor) to extract the AVSS.dll.new directly from the Haali Media Splitter installer executable and rename it to AVSS.dll (this avoids installation of the Haali Splitter, if you don't need it). However be aware that FFmpegSource2 requires Haali's Splitter too, so I recommend installing it!
Another source filter (Avisynth Plugin) that I can highly recommend is DGDecNV (DGIndexNV) by Neuron2. It is a Hardware-accelerated decoder for MPEG-2 as well as H.264/AVC and VC-1. Supported containers are TS, M2TS and also MKV (currently unsupported is MP4). DGDecNV has superseded DGMPGDec (DGIndex) and DGAVCDec (DGAVCIndex). Only drawback is that DGIndexNV requires a suitable Nvidia graphics card and you need to obtain a license for 15$. IMO well invested money.

Usage instructions:
The AVS Proxy GUI was designed to allow everybody to easily use Avisynth-input with Avidemux, especially people who aren't familiar with Avisynth yet. Therefor the GUI does not require the user to learn the Avisynth scripting language. The GUI will automatically generate a simple script, call the required source filter and load your input media file. Note that there is one Tab for each source filter supported by the GUI: FFVideoSource, DGDec(NV), DShowSource and AVISource.
Most of the time you will be happy using "FFVideoSource" (FFmpegSource2). It's 100% self-contained and hence doesn't require any additional decoders to be installed on your system (except for Haali's Splitter). Furthermore it supports a wide range of input formats (AVI, MKV, MP4, FLV, WMV, OGM, VOB, MPG, M2TS and TS). In case FFVideoSource fails, you may try "DirectShowSource" (or Haali's DSS2) as your fallback solution. But be aware that DirectShowSource relies on suitable DirectShow decoders and splitters to decode the input file (such as ffdshow and Haali Media Splitter).
For AVI files you may also use "AVISource", but suitable VFW (Video for Windows) decoders must be installed on your system! Most of the time decoding AVI files through FFVideoSource will work just fine and is easier to use than AVISource. For MPEG-2 and VOB files "DGDecode" (DGIndex) is the preferred source filter. And for AVC/H.264 streams "DGAVCDec" (DGAVCIndex) is the preferred source filter. Note that DGDecode and DGAVCDec need Index files as input! These Index files (.d2v and .dga) are created by using the DGIndex or DGAVCIndex programs. These are separate applications and you must create the Index files before you try use the AVS Proxy GUI. Also note that if you own a license of "DGDecNV" (DGIndexNV), then you should be using DGDecNV instead of DGDecode/DGAVCDec, as DGDecNV supports more Formats/Containers and also works much more stable. Moreover DGAVCDec is discontinued and thus has various known problems and limitations that have been fixed in DGDecNV. If you run into prolbems with DGAVCDec, either try FFVideoSource instead or purchase DGDecNV.
Advanced users may use the "Custom" tab to load or write their own custom Avisynth script. But be aware that the GUI will not check your custom script at all! If you encounter any scripting problems, refer to the Avisynth Wiki!

Feedback is welcome