What are some uncommon or "hidden" features of Interface Builder you wish you would have known about when first learning Xcode + Interface Builder?
Edit:
This question is specifically concerned with the GUI and not so obvious features of Interface Builder. And only tangentially related to Xcode.
I only found the Xcode User Defaults Reference the other day. It has a good number of things not available in the preferences UI.
You can use the mouse to measure the distance in pixels between interface elements: Select an element, like a button, then hover the mouse over another element and press option. This also works for subviews, like scrollers in a table view.
Sometimes it gets quite difficult to select an interface element that's buried in a deeply nested hierarchy of other elements (like a cell in a table column in a table view in a scroll view in the content view of a window).
Pressing shift while right-clicking on a view displays a nicely sorted stack of items to select.
After customizing a view or building a view hierarchy you can drag that view back to the library to save it in the current state. You give it a name and description. It will show up in a "Custom Objects" section and can be reused in other nibs.