Alan Lepofsky currently works in IBM's Business Partner organization, but has more than a decade of experience of working with Lotus Notes/Domino, primarily on the business side within Lotus. He currently maintains a Web site from which he likes to share some of the knowledge he has gained from using Notes and Domino.
Earlier this month, Lepofsky compiled a list of third-party application development tools for Lotus Notes/Domino, LotusScript, etc. The tools he lists include a Gantt chart for Lotus Notes, a reusable object library, a reusable Java class library for Domino and a Domino web application for browsing the data and designs of the Domino databases on your server, to name just a few. His list, which he makes clear is "unofficial," currently includes 57 tools, but the number is likely to grow since he has asked his visitors to add any he might have missed.
The entire listing can be found here on his Web site. Highly recommended. Check it out.