The First Slice was a demo version of portal freely available to owners of Nvidia graphics cards as a small promotion. Here I have recreated the demo as a sourcemod since the original is no longer available. Why you would want to get a downgraded version of the game even though you already own the full game, I don't know, but here you are!
Unless bugs are discovered, this will likely be the only release. If you want to use the maps in unpack, you can download the standalone maps folder from the "Addons" tab.
How to install:
1. Make sure you have Portal purchased and installed on steam
2. Extract the portal-first-slice folder into your sourcemods folder, you may have to make the folder yourself (C:\Program Files (x86)\Steam\steamapps\sourcemods)
3. Restart the steam client
4. The mod should show up in your steam library