An SDN denizen I see.
For my reccomendation:
Name- Journeyverse
Link
Type- Action/Adventure
Crossover- Lots. BTVS with SG1, Charmed, Highlander, Hercules, Aliens vs Predator, various cameo appearances, and more that I can't remember off the top of my head.
Description- Xander gets abducted by a Yautja on his road trip, taking a sentient magic weapon with him. Things get more complicated from there...
Reason for reccomendation- This thing is huge, and entertaining. It's well written, and although it does strain credibility a bit at some points it generally does a good job of integrating all the various universes together. The story behind it is also fairly entertaining, and there are a fair few bits of humour as well. People who like sci fi, BTVS style fantasy, or lost ancient civilistaions will like it.