I need to put up a easy to use. fast website. I love tumblr, but need something installable on my own server so that I can add my own custom functionality (seprate input forms, maps etc.).
I would like to avoid WordPress if possible, I really want to keep the posting UI as simple as possible for my users.
If you know php, built your own with: CodeIgniter. This is an AWESOME php framework which let you create very advanced php apps without too much work. Also there are many plugins available for CI.
else, i dont know, what is it exactly your looking for?
Check out this list of lightweight CMS options from WDL.
I have also used SkyBlueCanvas. It is very lightweight and not hard to figure out how to extend.
If you're worried about the complicated UI of WordPress (understandable), it is possible to create your own interfaces. See for example the work done by the WordPress.com team on the Prologue/P2 theme (available for free), which allows Twitter-style posting directly from the home page.