Yad (yet another dialog) is a fork of Zenity with many improvements, such as custom buttons, additional dialogs, pop-up menu in notification icon and more.

There were two main reasons to make this fork. The first one is to remove dependencies on deprecated libraries, such as libglade and gnome-canvas. And the second one - as for me, Zenity looks like an abandoned project. Its ChangeLog consists of just "bump version to..." and "translation updated" for the long time, but many interesting ideas which are ignored by developers/maintainers were in GNOME Bugzilla.

Example scripts on wiki

See more examples on wiki

Debian packages may be found here or here

For build YAD from svn use autoreconf -i && ./configure && make && make install commands

See NEWS for release changes