Half-Life 2 modder-turned-indie developer Richard Seabrook has begun taking pre-orders for Prospekt, a standalone Half-Life game made -- with Valve's approval -- using official Half-Life 2 assets.

It's a bit remarkable given that most game companies are typically reluctant to allow their work to be used by modders and developers in personal projects, even if those projects are non-commercial fan efforts.

In this case Prospekt is being sold on Steam as a $10 indie game, one that places players in the combat boots of U.S. Marine Adrian Shephard (the protagonist of Gearbox's Half-Life: Opposing Force) and pushes them through some of the events of Half-Life 2. 

Speaking to GamesN, Seabrook noted that Prospekt was borne as a job application for Valve. When the company failed to respond (Seabrook evidently mailed it to them in a suitcase spraypainted with the Half-Life logo) he put the game up on Steam Greenlight instead, and saw Valve approve it for sale three days later.

Since then, Seabrook has reportedly been in contact with Valve representatives who put him in touch with contacts to help finish the game and prepare it for release next month. 

He gave no details about what sorts of arrangements, if any, he reached with Valve to secure Prospekt's use of Half-Life 2 assets, though Valve will presumably take its traditional cut of the game's sales through Steam.