jKit is a very easy to use, cross platform, jQuery based UI toolkit, that's still small in size, has the features you need, and doesn't get in your way of doing things!
Put jQuery and jKit on all your pages and HTML becomes so much better. And the best thing? You really don't have to be a programmer to create a trully amazing website!
jKit has 99% of all the features you ever need. You don't have to check out dozens of plugins, learn how to use them, only to find out they don't work in a specific browser.
And even if jKit doesn't have that one feature you need right now, jKit is fully extendable with plugins and command replacements, all that and your API always stays the same.