Built on Eclipse, powered by JSF, and easy to use. Lotus Domino Designer integrates many development technologies within a common look-and feel IDE including Lotus Domino, Java, XML, and Web. It enables you to rapidly develop business critical applications and take advantage of new trends such as Web 2.0 and Service Oriented Architecture with a robust, scalable, security-rich platform. More >
Here is the link for the free download of the Eclipse based designer client for Domino 8.5.1.
Edited to Add: The free download actually is the all clients version with Notes, Developer, and Admin clients plus the Sametime client.