A Singleplayer Half-Life mod, which is actually excellent for a change. Could have been the Half-Life sequel, if Valve bothered to notice it.

NOTE: The following is taken from the official site http://www.poke646.com.

Story

It's 13 months after the incidents at Black Mesa. The Black Mesa Research Facility has been shut down. The nation never got to know anything about the experiments there. The less you know, the less you worry. The government founded a new organisation to handle "alien related topics". They hired the best scientists, technicians and military forces worldwide, its name is Poke646.

Some days ago the Poke646 scientists spotted alarming signals from the Xen homeworld. It looks as if the aliens plan to invade Nation City by opening a massive amount of portals. But the Poke646 Tech Labs are already prepared: some months ago the technicians developed a huge generator which is able to close hundreds of portals at once with a bundled energy beam. Four generators are about to be installed at the edges of Nation City. With all four activated they will form a giant beam and hopefully destroy the alien portal technology forever.

You play the role of Damien Reeves, the technical assistant of Dr. Fuller, and you don't know all these details. All you're currently worrying about is where the hell you lost your car keys when suddenly something hits your head and you slowly fade into unconciousness. When you wake up hours later, you quickly realize that some very strange things are going on...

Facts

Poke646 places the action in a giant city and lets you play through many thrilling environments, such as industrial districts, sewers, the city library, harbor docks, the subway, rotten bars, dubious hotels and many many more.

Although Poke646 is based on the HL engine, you will hardly find any resemblences with the original HL. Poke646 is all new, from the level architecture over the 20 Mb of new textures to the brand new arsenal of weapons. Additionally you'll face some new enemies and many unique gameplay elements (like the note system) which have never been realised in a HL mod before. To round it all up, the audio part of Poke646 shines with a suberb soundtrack, weighing in with 24 Mb alone. The tracks play for more than half an hour and deeply enhance the general atmosphere of the story.

Go Download it, as it is VERY good.