WiiDoom is a Wii port of Doom using the open source PrBoom engine.

Instaructions:
Twilight Hack method: Copy the prboom folder and boot.elf into the root directory of your SD card. Launch via Twilight Hack.

Homebrew Channel: Copy the WiiDoom directory (inside the apps directory) into the "apps" folder of your SD card. Copy the prboom folder to the root directory of your SD Card. Launch the Homebrew Channel.

Inside the prboom folder, you'll see prboom.wad and freedoom.wad. If you want to play "normal" DOOM (i.e. DOOM/DOOM II/Final DOOM), move freedoom.wad to another directory and copy the version you want into this folder (doom.wad, doom2.wad, etc..).

Once Doom comes up, you'll need to disconnect and reconnect the Nunchuk. This is a temporary bug, but seems to be common with Wii homebrew.