It is also multiplayer only - which means you will need to find a server running the game (mod) to be able to play against/with other people.

For some people, the lack of a suitable server might prove to be the biggest problem with this game.

The game Hotdog points to is free but you do require one of the source engine game such as Counter-Strike: Source, Half-Life 2, or Day of Defeat: Source.

You can see a list of these source engine games below: There are loads of these free modded games available via Steam, all of which are free.

Here's a full list of games the commercial games that you'll need to buy (only one) that meet this requirement with present price. Most of them are often offered at half price or more. I've seen a couple as low as 90% off.

£13.99 Counter-Strike: Source

£9.99 Portal

£14.99 Left 4 Dead 2

£6.99 Half-Life 2

£13.99 Team Fortress 2

£5.99 Half-Life 2: Episode Two

£5.99 Half-Life 2: Episode One

£12.99 Left 4 Dead

£5.99 Day of Defeat: Source

£16.99 The Orange Box (Contains five games including Portal, Half Life 2 Episode l, Half life 2 Episode 2, Team Fortress 2 and one other)

£2.99 Half-Life 2: Deathmatch

£52.99 Valve Complete Pack

£16.99 Counter-Strike Complete

£14.99 Garry's Mod + Team Fortress 2

£26.99 Half-Life Complete

£14.99 Counter-Strike: Source + Garry's Mod

£16.99 Source Multiplayer Pack

£15.49 Team Fortress Complete

Please note that Portal: First Slice and the bundled Half-Life 2: Deathmatch do not fulfill this requirement.

http://store.steampowered.com/app/17500/