The best place to start if you need help with a specific product is to contact the developer. All WHMCS Marketplace developers have both a website and support URL listed.
The OpenSRS Domains Pro module allows you to register, renew and transfer domains from an ever-expanding selection of TLDs, including many new gTLDs. This module supports domain locking and WHOIS Privacy for select TLDs, as well as WHMCS Domain Sync.
The ISPAPI Registrar Module connects WHMCS to your HEXONET account.
Advanced domain module for WHMCS - Offer over 1000 TLDs to your customers
ZA Domains CO.ZA WHMCS Module - Free Module for CO.ZA Domain Resellers and Domain Registrars
High-performance Domain Availability Checks using HEXONET's registrar API