Web UI, Mobile UI and Wireframe Kits are helpful for the designers because such kits help them mock-up the user interface. Such types of kits provide designers basic user interface elements that they need to create a model for a website or a software.
Today, we have come up with an exciting collection of some amazing web and mobile user interface kits especially for our designing fraternity. Feel free to share your opinion about this collection.
Browser Form Elements PSD

Simple UI Elements

Web kit interface layout pack

Wireframe Shapes

Wireframe Symbols

Blue Modern Web UI

Sleek Video Player

Modern Web UI Set

OmniGraffle Wireframe Stencils

Website Wireframes Kit for Adobe Fireworks

Fireworks Mini Web Wireframing Kit

Wireframe Stencils for Microsoft PowerPoint

Keynote Wireframe Templates

Small GUI Pack

Wireframes Magazine

Pwireframing: Paper Wireframing

How to Create Website Wireframes

Website Wireframes

Shades of Gray: Wireframes as Thinking Device

Web Form Elements

Eclipse Stencil for OmniGraffle

Best Practice UX Forms Stencil

Grid template for OmniGraffl

Free Sketching & Wireframing Kit

Photoshop Template for Blueprint CSS Comps

Photoshop button templates

Webdesigner kit

Mobile UI Kits
iPhone 4 PSD

Android GUI PSD

iPhone PSD Vector Kit

iPhone GUI PSD Version 4

Fireworks template for Android

iPad Stencil for Omnigraffle

iPad and iPhone Design

iPad GUI PSD Kit

All elements of Maemo 5 GUI in PSD

Sqetch, an Illustrator Wireframe Toolkit

ipad vector GUI elements

Android 2.2 GUI

IPhone App Wireframe Template

Android Widgets

Android Wireframe Tools

Photoshop Android GUI Set

Free ipad template

iPad Wall Presenter

HTC HD2 Smartphone vector

HTC G2

Samsung Galaxy Tab P1000

iPhone4 Retina GUI PSD

Ultimate iPhone Stencil

iPhone GUI as Rich Symbols for Fireworks

Illustrator template for iPhone design

Samsung i9000 Galaxy S

Motorola Droid 2

iPhone 4 GUI PSD

Mobility: A free set of mobile UI design elements

Free button collection

Windows 7 Taskbar

May I also suggest:
Touch Gesture Reference Guide
http://graffletopia.com/stencils/587
Handy for designing touch UI.
Wow, I'm truly impressed. There's so many kits here to use! Thanks for compiling all of these, #bookmarked
Fantastic list!! But how do you get your designs on to a real live iPhone? Simple, check out App in Seconds. Check out our blog at http://appinseconds.com/blog/
Since the ICS is starting to roll out, it’s also good for App designer to have the Android GUI Design Kit for ICS (in both Nexus S and Galaxy Nexus):
http://techietalkz.com/2011/12/13/android-ics-gui-design-kits-in-adobe-photoshop-format/
If you prefer to sketch the GUI on paper, I also made a printable wireframe sketching template:
http://techietalkz.com/2011/12/19/android-printable-a4-gui-sketching-kit-nexus-s-and-galaxy-nexus/
Enjoy!
Best web design UI Kit - Futurico
http://www.behance.net/gallery/Futurico-User-Interface-Pro/2650227
Now it's biggest patterns pack
Nice! These are really great. Wish I had known about these when I started. Though I don't regret investing in Keynotopia's templates :) They have been the most comprehensive I've seen yet and they keep making them better with every update! Highly recommend as a one-stop shop for both wire framing and pixel perfect (ready to be developed!) designs.
wow..!! thanks for this great collection!! i love it, really!! :)