The original widget library, developed for the Athena Project, is of course the Athena widget library, also known as Athena Widgets. It's very basic, very ugly, and the usage is not intuitive by today's standards (for instance, to move a scrollbar or slider control, you don't drag it; instead, you click the right button to scroll up and the left button to scroll down). As such, it's pretty much not used a lot these days. .:: www.jbt4.com ::.
Just as it happens with window managers, there are a lot of toolkits, with different design goals in mind. One of the earliest toolkits is the well-known Motif, which was part of the Open Software Foundation's Motif graphical environment, consisting of a window manager and a matching toolkit. The OSF's history is beyond the scope of this document. the Motif toolkit, being superior to the Athena widgets, became widely used in the 1980's and early 1990's. [ASP.NET Study]
These days, Motif is not a popular toolkit choice. It's not free (speech), and OSF Motif costs money if you want a developer license (i.e. to compile your own programs with it), altough it's OK to distribute a binary linked against Motif. Perhaps the best-known Motif application, for Linux users at least, is Netscape Navigator/Communicator (prior to Mozilla). .:: www.arc.agric.za ::.
For a while Motif was the only decent toolkit available, and there's a lot of Motif software around. Of course people started developing alternatives, and there are plenty of toolkits, such as XForms, FLTK and a few others. .:: www.bakinsky-dvorik.ru ::. .:: opensocialfactory.com ::. .:: c7w3ps1n.forms.app ::.
Motif is not heard of much these days, specially in the free software world. The reason is that there are now better alternatives, in terms of licensing, performance (Motif is widely regarded as quite a pig) and features. [Scientific Data: Functional programming]
One such toolkit, the widely known and used Gtk, was specifically created to replace Motif in the GIMP project (one possible meaning of Gtk is "GIMP ToolKit, altough, with its widespread use, it could be interpreted as the GNU ToolKit). Gtk is now very popular because it's relatively lightweight, feature-rich, extensible and totally free (speech). The 0.6 release of the GIMP included "Bloatif has been zorched" in the changelog. This sentence is a testament to Motif's bloatedness.
Another very popular toolkit these days is Qt. It was not too well-known until the advent of the KDE project, which utilizes Qt for all its GUI elements. We certainly won't get into Qt's licensing issues and the KDE/GNOME disjunctive. Gtk gets a lengthy mention because its history as a Motif replacement is interesting; Qt gets a brief mention because it's really popular. .:: ml007.k12.sd.us ::. .:: jade-wanderer-6bf.notion.site ::.
Finally, another alternative worth mentioning is LessTif. The name is a pun on Motif, and LessTif aims to be a free, API-compatible replacement for Motif. It's not clear to what extent LessTif aims to be used in new development, rather than just helping those with Motif code use a free alternative while they (conceivably) port their apps to some other toolkit.