I, like many other college students, use tumblr to procrastinate as much as possible. I've enabled endless scrolling. I've changed my layout four times in a day. And most of all, I've gained a weird group of followers on my account, and wondered why they're following me. (Most of them are high school friends though.)
c'est positif (optimistes.tumblr.com)
Possibly one of the happiest tumblrs I've found ever. I love it!
Humans of New York (humansofnewyork.com)
I know it's own thing, but you can follow it on Tumblr! I love seeing the pictures, and some of the stories are so inspiring. Definitely one of my favorite websites.
Doctor Who (doctorwho.tumblr.com)
Doctor Who is one of my favorite shows, and keeping up with it on tumblr is great as well. They do respond to all asks, and they also combine both fan stuff and real stuff so well.
The Mindy Project Style (http://themindyprojectstyle.com/)
Not run by Mindy Kaling or her production crew, but these guys go out and find whatever Mindy's wearing and either link it directly or find similar alternatives. There's also a couple for Zooey Deschanel and Zooey's show New Girl as well which are all great.
We Love Canines (fyeahcanines.tumblr.com)
Because dogs are awesome, and I love finding dog pictures.
The Daily Bunny (http://dailybunny.org/)
Similar to We Love Canines, but these guys post daily. Not to mention, I love bunnies more than everything else in the world.
If you're interested in following me on Tumblr, I'm at icedpeppermintmocha.tumblr.com. :)