DownloadMon, Mar 21, 2016
Click here to download the SI installer.
Or try here if the installer gives you trouble.
SI is a Half-Life mod. You need Half-Life installed to play it.
Extract the archive to your Half-Life directory in Steam.
When the mod is installed, your folder structure should look like this:
Steam/SteamApps/common/Half-Life ├── htmlcache ├── platform ├── si ├── valve ├── valve_hd