Awesome Portal 2 Sentry Turret USB Desk Defender Wants You to Bring It
The Portal 2 Sentry Turret USB Desk Defender is one such option. Yes, it's forty bucks at ThinkGeek, and that's kind of expensive for a novelty toy, but it's an awesome gadget (and looks to be small enough to wedge in a stocking -- hint, hint). It's also far less expensive than that other Portal Sentry Turret.
Armed with a motion detecting turret, the USB-powered Desk Defender opens up and warns intruders that it's about to bring the pain. The caveat? It doesn't actually decimate passersby, though it does pretend to shoot them with sound effects pulled directly from the game. Hey, it's less mess to clean up.
If you need actual projectiles to send a message, there's always the USB Rocket Launcher, also available at ThinkGeek (for $25).