This is a program I have written (am actually still writing). It attempts to conjugate Hebrew verbs, including exceptional forms (like hollow roots, weak roots and roots ending on He).
It is written in Prism (Object Pascal for .NET) and Windows Forms and runs on Windows only so far. A Mac version using Cocoa is in the making. While the program starts in Mac OS X now, the Windows Forms GUI just doesn't support right-to-left scripts on Mac OS X (or any platform except Windows).
You can download the current test version here. The source code will be available once the program works a bit better.
I am working on a Macintosh port. The actual conjugation library is a portable DLL.
