Half-Life was one of the first 3D shooters in which you could also move up and down. The game has several successors, including Half-Life Opposing Force and Half-Life Blueshift or Half-Life 2. It also served as the basis for the modification Counter-Strike, which is now also available as a standalone retail game.
This game can still be bought today at Steam. The uncut version is not available in Germany.
Very simple: Install from CD and apply patch 126.96.36.199. For Opposing Force you don't have to do an update anymore and as a base for CS you can use it. A crack is not necessary as long as you use a valid CD key. The only important thing when patching it is to use the correct language version. English patches do not work on the German HL version and the other way around. I don't know if there are bots for it. For newer Counter-Strike 1.5 servers it may be necessary to apply the 188.8.131.52 patch. With this the language version seems to matter again. It also replaces the old WON] master servers with the privately operated WON2 servers of the Steamless-Project. So there are server queries again, only not on servers operated by Valve, but on privately operated ones.
The game can be used in the LAN and with dedicated servers also via the Internet. However, there are practically none of them anymore. The former WON servers no longer exist. For servers the port 27015 (UDP) has to be forwarded.