Do you mean a blogging platform? If so Blogger and Wordpress could be considered alternatives to tumblr.
If you mean more like tumblr for images. Soup.io is the closest thing to tumblr.
I thought about starting a tumblr clone.
This is what it would look like:
I never went through with it though because Tumblr already exists. There is no need for a second one. Also I doubt anyone would use my website.