Verified answer

Yes you can. The Nikon system allows you to use almost any nikkor lens made after 1959 on any nikon f mount body. This means that you could use outdated non Af lenses on your D5000 or the newest top of the line.

It is better to the lenses before the camera as the sensor in the D5000, D90, and D300s is the same while the quality of the lens improves at a much better rate. Nikon's bodies above the D300s like the D700, D3, D3x, and D3s have full frame sensors meaning that the image capture area is twice as large so it should be paired up with a lens that gives it twice the amount of optic ability. Although you can use the lenses from you d5000 with a full frame camera you will get better results with a full frame lens. I personally use my lenses between top of the line bodies and low end dslrs with no problem...

As to the amount of lenses Nikon has everything from 6mm to 2000mm as well as macros, tilt shifts, and superzooms. so their is no shortage on the lenses you can buy for the camera.

Yes, you can use the lenses on cameras of the same brand - especially if you are referring to Nikon. Nikon range of DX lenses can be used on their full frame bodies like the D1 and D700 but you need to set them up in the configuration.

There are some limitations to the Canon range. The EF-S lenses cannot be used and should not be mounted on their highest end full frame camera bodies (Canon 5D, 1D)