price: from 4.99 GBP platforms: C64
Excellent unofficial Alien game
Fans of the Alien films will recognise the storyline, setting and Giger-like artwork. Organism is superbly-produced and probably outdoes the official games in all aspects.
It's available in many formats; disk, tape and download. There's a premium boxed package containing many goodies including a printed map / poster. (see the review, which is well worth a watch.)
For emulator and C64 mini users there's a digital version for 4.99, and for real PAL or NTSC C64 users, there's a tape / disk version from 6.99 (These are separate links from the product page I've linked to - you may have to hunt around for the version you want.)