Nothing is as strong (gives you more options to use) than IS Boxer.
The addon gets you very strong features like FTL (which can be a pain to configure, in other softwares).
That is a subscription model, at $50/year.

Keyclone is fairly basic, and easy to understand.
It lacks instant swaps and real-time mouse broadcasting, but otherwise has everything you'd want.
A one time fee of $20 (per computer) that is transferable.

Hot Key Net (HKN) is script based, but there is an example script.
This is free, and free is a good price.



If you're serious about boxing, I'd go with IS Boxer.
Five accounts is $900/year; two accounts is $360/year.
$50/year isn't much in comparison.