See also Notebook App.
I don't know whether the doc for iwidgets widgets are online.
BWidgets has one. Michael McLennan did one [L1 ]. Also Donal Fellows [L2 ]. RS used the BWidget "page manager" to create a vertical-tab notebook, which conveniently can hold e.g. 26 single-letter tabs.
[ Category Command | Part of incr Widgets ]