For Growth Hackers/Marketers:
Simple self-hosting at a decent price.
If you don't have this installed. Stop everything and do it.
Shorten your links, track your clicks.
The best way to schedule tweets/Facebook posts and continually improve your social media strategy.
Essential tool for screenshots. Higher resolution than the one built into OSX. Enables you to capture visible part of page, the entire page, or a selection.
Ever need to send something between your phone (iPhone/Android) and your computer and been frustrated there was no simple solution? Here it is.
Easily determine the color of anything you see online.
Hubspot's amazing tool to see if your email has reached someone, if they've read it, and what links they clicked on. Essential tool for marketers (or anyone sending important emails).
I explained Buffer in the social sharing section above. The Chrome extension makes the experience even better. It adds a "Buffer" link under each tweet on Twitter, let's you Buffer images you find online if you hover over them, and much more.
Six Pixels of Separation
A fantastic podcast by expert marketer Mitch Joel who brings on a new guest each week.
Smart Passive Income
Pat Flynn makes tens of thousands of dollars in passive income monthly and brings on guests like Tim Ferriss to discuss everything from starting a podcast to creating new revenue streams. I especially enjoy Pat's solo podcasts where he dived into building audiences and creating blog posts that resonate.
Specifically for getting started in startups:
Do More Faster
Learn from the successes and failures of Techstars startups. Required reading for anyone who is accepted into Techstars and for good reason.
The Lean Startup
An essential read by Eric Ries about how to rapidly test ideas and constantly seek validation for your business. Everyone who reads this will start constantly talking about "pivoting" their startup.
The Startup Owner's Manual
Steve Blank taught Eric Ries (author of The Lean Startup) and Stanford so he can sort of be looked at as the Grandfather of the Lean Startup movement.
If you are going to have a co-founder (you probably should), both of you must read this book. Noam Wasserman looks at millions of data points and interviews some of the most prominent successes and failures in the startup world.
The Four Hour Work Week
Tim Ferriss is the master of lifestyle design. His first book has helped thousands of people find better jobs, create passive income streams and do what is most meaningful for them by eliminating what isn't. Much of it is highly influenced by Vagabonding: An Uncommon Guide to the Art of Long-Term World Travel by Rolf Potts - another fantastic read.
A guide about what an internship should be like and how to find the right one for you.
Startup Metrics for Pirates by Dave McClure
Probably the most important guide for building a scalable (and investable) company.