Just like Team Fortress and Counter-Strike, this was once merely a Half-Life mod that Gabe Newell and his co-workers decided to put some extra effort into. It became a high-end standalone title, and brother, did they make a good job. Improved textures, mechanics and overall level design.
The game is pretty simple at its core; it's a elimination/free-for-all game where you have to use your surroundings, levels and weapon - the power disc - to claim yourself the big name of the game. Moving fast and swiftly between platforms in a blitz of your enemies is vital for succession in this game. It takes some time to learn, and even longer to master. But by studying your opponents, you'll pick up their tricks soon enough and give them a taste of their own medicine.
One thing that I love about this game is the amount of servers that are available, over a decade after the game's release. You'll never have to wait for a server without open slots, allowing you to easily invite your friends to play with you. Trust me, this game is much more fun with your friends. The more players who join, the better and more intense the game becomes.
In conclusion, I think this game is extremely underrated and truly deserves a sequel. Lots of Valve fans seem to have forgotten about this gem, and even Valve themselves don't feature it on their official website. But I'm living proof that this game is still taking breaths. Forgotten but not lost, this game deserves your love. It's a part of the CS1 Anthology and the Valve Complete Pack, so chances are that you already have this game and simply forgot that you have it. Otherwise, it's available for the low price of $4.99 here on Steam (50% off as I'm writing).
And yes, the graphics might not live up to today's standards. But when games like Minecraft sells millions of copies, I don't see how graphics are an relevant issue.
Oh, a final tip to those who'll try this game out: don't fall.
9/10 (Would buy this game for a friend.)