If you are just looking to test the waters or develop a custom theme/module, the official is your best bet.
Steer clear of "keygen" sites and "nulled" downloads. In the world of web hosting, your are worth far more than the $18–$20 monthly license fee. free whmcs license key top
If your budget is truly zero and you don't need a reseller bundle, consider "Top" alternatives that offer a free tier. While not WHMCS, they provide similar automation: If you are just looking to test the