Just so that people know that they exist, I've uploaded two plugins.
This displays a single selection, multiple selection or columned list box (with column headings and optional check boxes). The plugin takes its list box items from the NSIS stack. I wrote this plugin as even with the makensis 8192 string length build I felt it was unsafe when having a large number of list box items for InstallOptions list boxes (from an Internet source).
A small 3KB dll that allows you to catch click events of any custom buttons on any NSIS dialogs. The buttons must be added to the UI files with Resource Hacker. The example included makes use of a custom button on the parent window (alongside the Next and Back buttons) and one on the Components page.