The jQuery team are approaching the UI 1.5 release, the library of demos is steadily building up and i personally cannot wait until its fully released . One of the “weaker” points of other Javascript User Interfaces and previous releases were the themes for the window borders etc. This has been solved in jQuery UI 1.5 with the theme roller. Its a well polished theme roller with features like the jQuery colour picker in it, you set the necessary classes and you can see a live demo of what it will look like on dialogs, sliders and tabs below, very nice.

Heres a video by the Filament group, who made the theme roller.

ThemeRoller for jQuery UI Screencast v4 from filamentgroup on Vimeo.