We used
Laserglow.com's 2.4 Watt Laser Projector that we were lucky enough to have out our disposal. Naturally, you can use whatever you'd like for the star/angel, but I would suggest using something that appears drastically different from the rest of the tree. If you are using primarily green and red lasers, than a yellow or a blue might be a good idea for your star. If you happen to have a yellow or a blue laser at your disposal than try it with a
diffraction grating pointing up and you should be able to create a unique star for the top of your tree.