The current beta version is 3.0b3.
Source is available at ftp.x.org .
This web page is not entirely up to date with respect to version 3.0 features. The previous stable version is 2.0, also available at ftp.x.org . Due to my involvement with the LessTif project , it only operates as a child of an XmScrolledWindow. This version only draws items that are shown in the window, rather than rendering the entire list while clipping to the visible window. All features from version 2.0 have been retained, and many new features have been added.