I recently did some work making a pretty simple Tumblr app.
I used the Tumblr API to authenticate a user login, retrieve their avatar, number of posts, username, and the last 10 posts made by them. The other option I give is to search for a Tumblr site by name, for instance, searching for ‘levijackson’ will pull in the two posts I have ever made on the site 
Right now there isn’t much validation of input, so it may be wise to add in some for the login form and the search box.
Local storage
Something that I had been dying to play with was local storage! I finally got a chance to do so with this and I must say, pretty snazzy. I enjoyed being able to access the data using methods/object notation instead of needing to use a third party or home brewed Javascript cookie function.
Graceful degradation
Having used local storage, I didn’t want to exclude any browser that does not have local storage capabilities, and so I stored some of the information as a cookie as well for the browsers that do not use local storage. In addition to both of these methods, I cached the content I retrieve from Tumblr in text files so as to not pound their servers with the tons of traffic I get daily.
The end
Anyway, you can test it out or download the code and modify it all you want.
If you spot anything glaringly broken on it or have any suggestions of where to go next with it, let me know! I realize it is far from perfect, but when used right it does work well.