— AppCooker — http://www.appcooker.com

Prototype editing: iPad
Prototype viewing: iPad
Prototype hosting: n/a
Widget library: yes
Price: $24.99

— App In Seconds — http://www.appinseconds.com/

Prototype editing: web app
Prototype viewing: mobile web app
Prototype hosting: Fully hosted
Widget library: no (based on full screen mockups)
Price: $9.95/month or $99/year, free trial available

— Axure — http://www.axure.com

Prototype editing: Mac and Windows apps
Prototype viewing: Via AxShare
Prototype hosting: n/a
Widget library: yes
Price: $589 each for 5 copies, $149/year for updates

— Blueprint — http://www.groosoft.com/blueprint/

Prototype editing: iPad
Prototype viewing: iOS Blueprint viewer
Prototype hosting: n/a
Widget library: yes
Price: $19.99 (lite version is free and limits to 2 project and 8 screens)

— Codiqa — http://codiqa.com

Prototype editing: web app
Prototype viewing: mobile web app
Prototype hosting: Fully hosted
Widget library: yes
Price: $10/month, $25/month, free for editing only

— Fieldtest — http://fieldtestapp.com

Prototype editing: web app
Prototype viewing: mobile web app
Prototype hosting: fully hosted
Widget library: yes
Price: not available (private beta)

— Fluid UI — http://fluidui.com/editor

Prototype editing: web app
Prototype viewing: mobile web app
Prototype hosting: fully hosted (Self-hosting options planned)
Widget library: yes
Price: Free account, $29/month, $49/month and $129/month

— iMockups — http://www.endloop.ca/imockups/

Prototype editing: iPad
Prototype viewing: iMockups presentation mode
Prototype hosting: n/a
Widget library: yes
Price: $6.99

— InVision — http://www.invisionapp.com

Prototype editing: web app
Prototype viewing: mobile web app
Prototype hosting: fully hosted
Widget library: yes, free UI library items available for download
Price: FREE for 2 projects, $25/month, $75/month

— iPrototype/TAP — http://unitid.nl/2011/03/touch-a…

Prototype editing: Adobe Fireworks
Prototype viewing: mobile web app
Prototype hosting: self hosted (requires server setup)
Widget library: yes (as Fireworks documents)
Price: free

— iRise — http://www.irise.com

Prototype editing: Desktop client (Windows and Mac)
Prototype viewing: Browser-based emulator and on-device (a free native iOS app — iRise Mobile — is available in the iTunes app store)
Prototype hosting: Collaboration server (Definition Center) in Enterprise edition
Widget library: yes
Price: Varies by version – Studio MX is free – http://www.irise.com/products/ir…

— jQueryMobile — http://jquerymobile.com

Prototype editing: hand coding
Prototype viewing: mobile web app
Prototype hosting: self hosted
Widget library: yes (there are some UIKit-like javascript libraries, in code form)
Price: free

— justinmind.com —  http://www.justinmind.com

Prototype editing: mac or pc app
Prototype viewing: mobile web app
Prototype hosting: not sure
Widget library: yes
Price: $499, $99/year for updates and support

— Keynotopia — http://keynotopia.com

Prototype editing: Apple Keynote
Prototype viewing: PDF viewer or native Keynotopia iOS app
Prototype hosting: downloaded PDF
Widget library: yes (as Keynote templates)
Price: Keynote templates start at $49, presumably you can use your own; iPhone viewer is $1.99, iPad viewer is $3.99

— Mockability — http://www.mockabilly.com/

Prototype editing: iPhone app
Prototype viewing: iPhone app
Prototype hosting: local on phone
Widget library: no (based on full screen mockups)
Price: $6.99, free lite version limited to 2 mockups with 6 screens

— Proto.io — http://proto.io

Prototype editing: web app
Prototype viewing: mobile web app
Prototype hosting: fully hosted
Widget library: yes
Price: $0/month (free plan), $24/month, $49/month, free 15 day trial

— ProtoShare — http://www.protoshare.com/

Prototype editing: web app
Prototype viewing: mobile web app
Prototype hosting: fully hosted
Widget library: yes
Price: $300/year, $750/year, free 30 day trial

— Prototypes — http://prototypesapp.com

Prototype editing: Mac app
Prototype viewing: mobile web app
Prototype hosting: fully hosted
Widget library: no (based on full screen mockups)
Price: $39.99

— Realizer — http://realizerapp.com

Prototype editing: web app
Prototype viewing: iOS app
Prototype hosting: fully hosted
Widget library: no (based on full screen mockups)
Price: free

— Refine.io — http://refine.io

Prototype editing: web app
Prototype viewing: mobile web app
Prototype hosting: fully hosted
Widget library: yes
Price: not available (private beta)

— MockingBot — http://mockingbot.com

Prototype editing: web app
Prototype viewing: web app
Prototype hosting: fully hosted
Widget library: yes
Price: $0/month (free plan), $7/month, $15/month

Additionally the following tools don’t let you build prototypes per se, but let you preview the on-screen contents as you edit them.

— LiveView — http://www.zambetti.com/projects…
— Skala Preview — http://bjango.com/mac/skalapreview/
— xScope — http://xscopeapp.com/

http://www.endloop.ca/imockups/
http://interface2.lesscode.co.nz/
http://www.realizerapp.com/
http://www.appcooker.com/ ($19,99)
http://www.omnigroup.com/product…
http://www.groosoft.com/blueprint/
http://www.applicationcraft.com/
http://dapp.kerofrog.com.au/
http://www.mockflow.com/signup/ ($69 per year)
http://www.balsamiq.com/ (web based)
http://creately.com/ (web based)