UI Guidelines:
iPhone, iPod Touch, and iPad. Big collection of these available here which I need to parse out http://www.queness.com/post/10870/useful-collection-of-ios-tools-and-resources-for-designers
Designing for iOS7 from developer.apple.
iPhone Human Interface Guidelines from developer.apple.
iPhone Human Interface Guidelines from developer.apple. (PDF)
iPad Human Interface Guidelines from developer.apple.
The iOS 7 Design Cheat Sheet from Ivo Mynttinen.
iOS UI Element Guidelines which despite other tools I use several times a week... well, I used to but the iOS 7 update messed it up pretty bad.
Stencils & Templates:
iOS7 GUI PSD (iPhone) from Teehan-Lax (Photoshop).
iOS GUI for Sketch from Teehan-Lax (Sketch).
IOS7 GUI Fantastic Set With More Than A Thousand Elements, from DesignShock. (Photoshop)
iPhone 5 Grid a 10-column
grid for comps. (Photoshop) iOS 7 Wireframe Kit in B&W, for classic wireframing. (Illustrator)
iOS 7 WIreframe Kit for iPhone, from Funsize. (Keynote)
Ipad Retina Mockup (Psd) from blugraphic. (Photoshop)
Library of iOS7 UI Elements (Photoshop)
Library of iPhone UI Elements Photoshop library of iPhone-specific elements only, from Teehan-Lax. Updated occasionally, so if you see a new one, please update the link for us. (Photoshop)
http://www.teehanlax.com/blog/2010/08/12/iphone-4-gui-psd-retina-display/ Same as above but for the iPhone 4 resolution instead. (Photoshop)
iPhone UI Kit Lots of UI widgets like switches, buttons, etc. (Photoshop)
Another Libtary of iPhone4 UI Elements Based on the Teehan-Lax work above, but presumably different in some way. (Photoshop)
Vector Wireframe Templates for iPhone (Illustrator)
iPhone 4 HD Another one, not sure who made it or if it's fresh or a derivation of the Teehan-Lax stuff again. (Photoshop)
iPhone Wireframe Kit In, of all things, Google Docs. Haven't tried it, but I really need to. (GoogleDocs)
iPad Template Preview only has the frame, so not sure what is included. (Photoshop)
iPad Wall Presenter I think just the frames with guides to put backgrounds and designs into. (PNG and Photoshop)
Sketching the iPad from Zurb to encourage sketching, even as far as to print empties and use your mad Sharpie® skills. (OmniGraffle)
iPad and iPhone Design by da5id, via Graffletopia. (OmniGraffle)
iOS Application Icon Template from The Icon Handbook. (Illustrator)
iPhone & iPad PSD templates & icons by Rafi (Photoshop)
MOObileFrames iOS Wireframe Tool from Raiz Labs. (Keynote, Powerpoint)
iPad Template for Visio from IconSlinger. (Visio of course)
UI Pack for iOS from rebirthPIXEL. Lots of faux-leather and other skeuomorphic textures. (Photoshop)