Ireland-based RealView Innovations is going to give a little more depth to PSP games this holiday season, with the launch of its new peripheral, the V-Screen. According to the press release, the device "enhances the perceived depth of the 2D screen image." Of course, it also makes the PSP significantly less portable, so you have to be pretty committed to kind-of-3D imagery to use it.
On the other hand, the V-Screen (as far as we can tell) doesn't require the user to get right up to the PSP like the other 3D attachment, reducing both neck strain and resemblance to Spock. Also, unlike the Solid Eye, the V-Screen isn't designed to work with just a single game.