I recently came across some good UI sets but this is one of my favs. Bruce Galpin created a great Google UI set that includes buttons, dropdown menus and a lot of icons that work great in your next projects.
The pack provides Google+ Action Buttons, Google+ Icon Buttons with Label, Dropdown Menu Buttons and over 200+ Google+ icons. Every icon has hover stat which works great and most modern browser. The compatibility of of this Google UI set is set to Firefox, Chrome, Safari and IE7+.
The pack is under Creative Commons Attributions and you are free to use them in commercial projects, modify them, adapted them or anything that comes in your mind.
Google+ UI pack preview