This extreme 650-1300mm f/8-16 Manual Focus Lens from Bower is an economical way to achieve some ultra-telephoto photography, all the way up to 1300mm.  This is a T-mount lens, which means that aperture is not communicated to or controlled by the camera body, and as such must be used only in aperture-priority auto-exposure mode (if your camera has it) or in the manual exposure mode.  There are T-mounts available for every modern DSLR system as well as for older film cameras and even C-mount cameras.

It will work on both "full frame" digital cameras as well as the smaller-format APS-C (15 x 22mm) cameras, but has only one effective aperture at any focal length, which varies from f/8 at 650mm to f/16 at 1300mm.  That's not much of a hindrance, however, as you'd likely use this lens at its relatively small maximum aperture in any case.

This lens requires bright light and a high ISO setting on the camera to get a fast shutter speed (or fast film in a film camera), and needs a sturdy tripod or monopod, but can deliver very sharp results if used properly.