As for today, the famous RAW converter Rawshooter still does support .CR2 files from the Canon EOS 400D/XTi.
If you are a Rawshooter user and own a Canon EOS 400D here is a quick hack to solve this problem.
1) Convert your .CR2 to .DNG: Download Adobe DNG converter 3.6 and convert the raw file of your Canon EOS XTi .CR2 to .DNG
2) Hack RawShooter.exe: Make sure to make a backup of your RawShooter.exe file then open the file using any Hex Editor. Find and replace each instance of ‘350D’ with ‘400D’
N.B:Make sure the new file has EXACTLY the same number of bytes as the original.
Well you’re pretty much done, just run the RawShooter.exe file you’ve just modified and it should be able to open all the .DNG files of your Canon EOS 400D.
Only drawback is that you won’t be able to open or edit file from a Canon EOS 350D / XT using this version of Rawshooter.exe, if you need to just use the original .exe file you’ve previously backed up, should work like a charm.
Thank you PhotoHans for the tip.