With the release of Unity 2.6, we now have a nice way of syncing to Visual Studio projects. This opens the door to some nice IntelliSense for both MonoDevelop and Visual Studio. What is still missing from the picture, is some inline documentation to go with it. That is the void, to which these packages fill. Currently only C# is supported, I'll make sure to come up with something when UnityScript comes to MonoDevelop/VisualStudio.
If anyone has any suggestions which are feasible (meaning the app has to have a method for inline docs), let me know (Reapazor) and I'll see about generating files for it. Or if Unity Technologies would be so kind as to give me some access I would love to make the official docs.
Visual Studio Screenshot
It's in the works, but a stripped down version of MonoDevelop.Unity will get installed into MonoDevelop to add shortcuts to get to the Unity documentation.
As people do not like running "random" installers, I will tell you ALL that this installer does is move the compiled MonoDoc sources into the MonoDoc sources folder (located under /Library/Frameworks/Mono.framework/....). It also installs a small addin into MonoDevelop which allows you to search to documentation of Unity. (Key Bindings are under MD's own Key Binding Settings)
Mac Download v003
Supports: MonoDevelop, Visual Studio (2005,2008,2010)
This installer copies the MonoDoc sources into the sources folder, located under the Program Files\MonoDevelop folder, as well as puts the VS XML files into the Unity\Editor\Data\lib\ folder.