The first toolkit developed for X. Tricky to program. Difficult to use (mostly because keyboard focus strictly follows the mouse; you have to be pointing to a widget to be able to type into it.) Ugly too. Existed mainly as a proof-of-concept. Allegedly. Even Motif is much better.
There are many examples of Xaw (as it is commonly known) applications about: xterm is the best known, and xfig is probably the nicest (about which it is possible to say "nice app, shame about the toolkit".) Most of the others are either not in common use these days or are so simple that its not worth replacing them (unless you're implementing a whole desktop like Gnome or KDE.)
There's even a 3D version available (Xaw3d) but while that fixes the look somewhat with intelligent resource DB choices, it's still not a usable toolkit (unless the developer has put a horrendous amount of thought into the Xt-equivalent of bindings.)
OTOH, Xaw doesn't look massively worse on monochrome (i.e. single-bit-deep) displays, unlike virtually every other toolkit in existence on the planet. But who has monochrome displays these days?