Tumblr just launched a weirdly bare-bones app for Apple's new OS X Yosemite. If you have a Mac and you download it, you'll be able to post to Tumblr from elsewhere on the web using a "Share" button.
The other thing it does: You can also use it to turn your computer into a Tumblr-only zone. "You can even make it full screen, so that your computer displays 0% non-Tumblr content," the app description reads. That seems like overkill, but I can admire their confidence that people will actually want to filter their entire browsing experience through Tumblr.
This app will appeal to the segment of heavy Tumblr users ("Tumblheads") who primarily post on desktop; there are already iOS, Android, and Windows Tumblr apps for those who want to Tumbl on their phones.
You can also just use Tumblr on your browser and look at other stuff on the Internet too.