Sorry jQuery, Prototype, Yahoo YUI, Dojo don't appeal to me in when considering small. I wanted a library that would be modular has the smallest code possible upto 20Kb[un compressed] is what i expect. should provide Dom manipulation[no ajax and badass stuff] and can be extended, is widget friendly.
There are quite a few small javascript libraries on microJS. You may be interesting in taking a look at $DOM (2.6kb).
Alternatively, if you want a bit more functionality while still being small, you can check out XUI (10.4kb, or 4.2kb gzipped).