1. Unlocking Your Phone

As of January, it's illegal to unlock your phone to use across various carriers. However, jailbreaking, or the process of removing limitations on iOS to allow for certain downloads and customization, is still technically legal.
Even when your contract with a specific carrier expires, it's still unlawful to unlock your phone by yourself using third-party methods, under a new ruling from the Library of Congress in January. If you want to switch carriers, authorized unlocking by an authorized carrier is still legal.
Basically, you can't unlock your phone yourself, but you can let AT&T or Verizon do it for you legally.

2. Ad Blockers
Though they may be annoying, those pesky banner, pop-up and in-stream ads that crawl the web are (surprise!) actually paying for the content you are viewing for free. By bypassing these ads, it's almost like not paying the fare to view the content, and in some cases, ad-blocking may be illegal. For certain sites, there's potential for billions of dollars in losses each year, due to visitors not actually seeing advertisers' messages.

3. Making GIFs and Memes

Believe it or not, it's illegal to use copyrighted material without permission. Though it's legal to parody copyrighted material in the vein of criticism, lifting copyrighted material straight from the source is illegal. However, it's almost impossible to prosecute GIF-making (looking at you, Tumblr users) because of its wide distribution and murky original sourcing.

5. Downloading Files and Using Without Permission

An obvious one, it's actually illegal to just pull any old JPG, PNG, PDF or any type of file you'd find from a simple Google search. It's fine to view them online as is, but once you download that image of a cat you're Googling, you have the power to distribute it to your liking, and that's where it becomes illegal. The exemption, of course, is if you are granted specific permission from the owner of the file, or if the material has a fair use license.

6. Connecting to Unsecured Wi-Fi networks

Look over your shoulder the next time you hop on that Starbucks Wi-Fi from across the street. Even though the network's there for the taking, authorized usage of Wi-Fi networks could be counted as illegal under the Computer Fraud and Abuse Act, which applies to wireless routers.

8. VPN and IP Address Loopholes
Abroad and can't stream Netflix? Screwing around with VPN to make your IP address look like you're in America is one way to circumvent this issue. Of course, it's not technically legal, and the urge to binge-watch TV shows doesn't help either.

Lol

Guilty

We are all guilty then

9. Sharing Subscription Passwords

Of the millions who subscribe to HBO Go and Netflix, about one-third admit to having shared passwords before. Freeloading on these services isn't hard, and in fact, studies have shown that password-sharing has helped bring new customers to these companies.

10. Bypassing Paywalls

Though it's as simple as a few Google proxy servers, bypassing paywalls is just another way to get content you're supposed to pay for without actually coughing up the cash.

11. Uploading Copyrighted YouTube Content
Probably the oldest hack in the book, uploading copyrighted material to YouTube is, you guessed it, illegal. But when there's just too much for YouTube to handle, you're still bound to find that '90s Simpsons rerun somewhere in the depths of YouTube -- before mods take it down, that is.

cc: skydancer, Maleeq