Default Widget Styles
tdebase includes several widget styles: CDE, KDE Classic, keramik, MS Windows 9.x (AKA "Redmond"), Motif, Plastik, Platinum, and SGI.
tdeartwork includes two additional styles: .NET and Phase.
In addition, there are some styles among the Applications packages (currently two: qtcurve and lipstik).
Visual Comparison of Styles
(not yet created)
Using Non-Trinity Widget Styles with Trinity
Unfortunately, this doesn't work very well. Widget styles in Trinity involve code as well as graphical elements, and that means that they can be affected by API changes. Widget styles created for KDE3 will not work in Trinity without a conversion process.
That being said, if you have a favourite KDE3 style that you just cannot live without, you can request that it be converted, either by filing a bug or, if this proves impossible for some reason, posting a message to the Users or Developers mailing lists.
Widget styles made for desktop environments other than KDE3 won't work even with a conversion process--essentially, they have to be recreated from scratch.