Tunnels & Trolls has its roots in the sword and sorcery genre. Having been written in 1975 at the very beginning of RPGs this is to be expected. But the basic mechanics of T&T are very flexible and can be adapted to work in a variety of genres from old west, gangsters, modern, superheroes, cyberpunk, space opera, and horror. The only thing needed are a few extra rules and equipment to cover the world you are creating. Luckily several members of the T&T community have already done this and there are many rules supplements available to tale your T&T game in completely different directions.
Peryton Publishing. The game is set in the far future of Trollworld, where magic and technology have developed side-by-side. New Khazan includes modifications to the standard character types and kindred plus details on kindred special abilities, some changes to the magic system, rules for building spaceships and space combat (of course), a large list of sample ships, and new equipment. There is also a complete description of the 9,000 Worlds and a number of adventure seeds to get you going. There have also been a handful of GM adventures and even solo adventures published to support New Khazan. You can get a PDF copy of the rule book through RPGNow for $7 or a print copy through Amazon for $18.
Wizardawn in the vein of Gamma World and similar games. Mutants, lost futuristic technology, rogue robots, and aliens abound. The book includes a vast array of abilities and mutations, modifications to character kindreds (human, mammal, reptile, insect, plant, etc.) and types, new equipment, rules for scavenging, artifacts (laser rifles and power armor anyone?), vehicles, monster creation, treasure tables, and ideas for designing your own post-apocalyptic world. Best of all this supplement is available for free from Wizardawn Tabletop Games.
RPGNow for $10 or a print copy through Lulu for $15 (a bargain given the size of the book and quality of the art).
Jerry Teleha. While it has been most strongly associated with zombie apocalypse type games (especially given the prevalence of interior zombie art), Stay Alive! is suitable for any type of modern horror game featuring vampires, werewolves, demons, crab people, cultists, or whatever. Of course modern does not simply mean today's world but can include pre-industrial, late 19th century, early twentieth century, or future worlds. The current version of the book is labeled as a "Lite" edition and includes modifications for character kindred (humans only), character types (citizens and warriors only), and character attributes, new equipment including modern firearms and armor, and rules for handling firearms in combat. Stay Alive! is available in PDF format at RPGNow for only $1 or as printed book through Amazon for $6.
There are certainly a lot more options for new setting and new worlds to use with the T&T rules (giant robots anyone?). These four supplements represent a perfect starting point for a lot more development. I'm also looking forward to seeing some additional support material for these settings, especially solo adventures.