- Sofa: http://www.madebysofa.com/
- Pinvoke: http://twitter.com/pinvoke_com + http://www.pinvoke.com
- ByLine Break, Midtone Design: http://www.midtonedesign.com/ + http://twitter.com/DDrDark + http://twitter.com/bylinebreak + http://bylinebreak.com/
- Cocoia: http://twitter.com/Cocoia + http://www.icondesigner.net/
- Louis Harboe: http://twitter.com/spiralstairs + http://www.graphicpeel.com/
- Jonas Rask (retired): http://twitter.com/JonasRask + http://jonasraskdesign.com/
- Mathieu White: http://twitter.com/MathieuWhite + http://mathieuwhite.com/#funtime
The design company is run by Everaldo, the creator of the Crystal Project http://www.everaldo.com/
His icon sets have been some of the highest rated and most downloaded at kde-look.org for years now
If you are not familiar with kde-look.org, check it out. There are hundreds of very impressive icon sets you can download for free. Many are released under LGPL and can be used in commercial software
Crowdspring even guarantees 25 entries, worth checking out
They've done the icons for Coda, Windows XP, and others I believe.