Because I started works on UI interface, I need some toolbar icons. After a lot of search, I found few sets which are nice and free, and few which are nice and paid ;-).
Here is compilation of UI sets links:
http://blog.twg.ca/2009/09/free-iphone-toolbar-icons/ (160 icons for free)
http://glyphish.com/ (130 icons for free, next 80 paid)
http://smykes24.deviantart.com/art/iPhone-Tab-Bar-Icons-Gaming-129467506
http://tabs.icondesign.dk/ (few for free, rest of them paid)
http://www.app-bits.com/free-icons.html (64 icons for free)
http://www.dezinerfolio.com/freebie/30-free-vector-icons (30 vector icons for free)
http://www.iconsweets.com/ (60 free vector icons)
Paid icon sets:
http://www.icon2u.com/icon
http://www.iconsberlin.com/index_black.php (428 icons for 50 €)
http://www.iphonicon.com/ (260 icons for $79)
http://www.pixelpressicons.com/?page_id=118 (350 icons for $69)
http://www.eddit.com/shop/iphone_ui_icon_set/ (160 icons for $69)
http://www.kombine.net/icon-store/iphone-tab-bar-icons (130 icons for $60)
http://www.designkode.com/iphone-ipad-tab-bar-icons (150 icons for $50)
http://www.designkode.com/iphone-ipad-tab-bar-icons (1000 icons for $75)
I hope you can add http://www.IconBeast.com into the list as well. Their iOS icon is worth a mention.