A shareware game by PomPom. Space Tripper is a very flashy 3D variant of Defender. From the manual (reproduced verbatim... showing the deplorable standards of literacy so common in shareware circles - twajs):

Space Tripper is a frantic Shoot-em-up bought to you from people who remember what Shoot-em-up means!!
Fly your ship through 14 arenas, with your only mission Being to kill everything within a time limit. Only then can you proceed to the next stage. Be warned... this game holds no punches. This is one seriously tough Shooter!!

Each horizontally-scrolling arena is presented from a birdseye view. Your craft can only face left or right. Enemies teleport into the arena in waves. Everything is in very polished full 3D, with a graphical style broadly similar to Virtual On (brightly coloured, highly angular ships and robots). The game requires very finely honed reflexes. It's ruthless.

The only downside with the game is that in spite of the gloss there is very little to do, in fact the game is substantially more simplistic than practically any other scrolling shooter (even Defender). The weapons (at least as far as I've seen) are firmly of the peashooter variety as well. I like shmups where you can blow things up with abandon, not slowly pick the health off of each lowly grunt. Still, it's shareware so you can try before you buy. More info: www.pompom.org.uk