RBSplitView address limitations in Cocoa's NSSplitView, if you need to limit subview's sizes, expand or collapse subviews programmatically or by double-clicking, or resize the split view frequently.
RBSplitView has special content views - RBSplitSubviews - that handle details of subview limitations and properties. So there's less or no work to be done by the delegate. RBSplitView also has built-in support for nesting any number of levels, and automatically generates a two-axis thumb to resize in two dimensions.
The download includes the basic RBSplitView and RBSplitSubview classes, an Interface Builder palette, full source code with Xcode project (for version 2.4.1), full documentation, Universal prebuilt library and framework in both debug and release versions, and a sample application showing most features.