Wireframing
Balsamiq: a wireframing and mock up tool with a high focus on usability (instructions for getting started)
InVision: transform your Web & Mobile (iOS, Android) designs into clickable, interactive Prototypes and Mockups (instructions for getting started)
Proto.io: create fully-interactive high-fidelity prototypes that look and work exactly like your app should. No coding required.
Landing pages
LaunchRock: a website builder dedicated to early customer acquisition. Publish a landing page in minutes and validate your startup with over 20 million early adopters (instructions for getting started)
Unbounce: lets marketers build, publish and test landing pages without IT or software (instructions for getting started)
KickoffLabs: quickly & easily setup refer-a-friend giveaways, sweepstakes, and product launches (instructions for getting started)
SITE/App-BUILDING TOOLS
SquareSpace: all-in-one solution for creating a website (instructions for getting started)
Wix: create a free website (instructions for getting started)
Strikingly: website builder -- quick, easy, stylish, mobile-friendly
Bubble.is: makes programming accessible to everyone without code
Online ad Campaigns: getting Started
Google AdWords: Set Up Your Google AdWords Campaign in 9 Steps; Your Guide to AdWords; Google AdWords Made Simple; How to Set Up AdWords
Facebook Ads: How Facebook Ads Work; The Complete Guide to Getting Started With Facebook Ads; Beginner's Guide to Advertising on Facebook; Entrepreneur.com Guide to Getting Started With Facebook Ads
testing
Optimizely: test and personalize your website (instructions for getting started)
Validately: recruit testers, launch tests, and analyze results. HBS alumni founder will allow you to recruit panelists for half the normal price -- ask Prof. Austin for details (instructions for getting started)
Presentation design
Powtoon: free tool to create animated videos
Slides Carnival: free presentation design templates
Unsplash: free high resolution photos
Smartmockups app (free trial available): tool to create stunning product screenshots without using Photoshop
Other resources for Finding tools
Steve Blank's Startup Tools
Startup Stash, a curated directory of 400 resources & tools for startups
Startup Toolkit by Thomas Oppong