The following tools are very helpful for installing zip lines.
Cable Puller Tools
A cable puller will be necessary for medium to long zip lines because it is the best way to tension the cable and hold it in position while you make the last end connection. Without the right tools, you and or a couple other adults will be trying to hold the zip line cable tight while you tighten the clips. This is very tiring and probably impossible on zip lines over a couple hundred feet. You'll waste so much time without it that it should be considered an essential tool.
Cable Grip Tools / Klein Haven Grip
This tool is often used in conjunction with the Cable puller above. You can easily put the cable grip onto one end of the cable puller and tie the other end to the terminating pole/tree and now you can tension your zip line. The cable grip can also be used to tie off a cable on it's own, without a cable puller. These two tools are the favorite of professional zip line installation companies. One such small company told me that they own over 20 of them and take at least 4 to each zip line installation. This Klein Grip works for zip lines with up to 1/2" cable.
Tree Slings / Tools to Wrap Trees while Pulling
While the Klein Have Grip goes on the cable end of the cable puller, the other end has to attach to the terminating pole or tree. When using a tree, you should take care not to damage the bark while during zip line installation. This tree sling is wide enough not to cut into the bark during use. However, this tree sling is only an example - there are others that will work just as well for installing a zip line.
Cable Cutters & other Tools to cut Zip Line Cables
Cutting large diameter zip line cable is easiest with a nice set of cutters with long handles like this tool. Other tools will work for cutting zip line cable, such as grinders, hacksaws, sawzalls with metal blades, etc. But this is the kind of tool used by most zip line builders.