Twittme is so mobile it does not even have a standard webpage for information. The UI is simple and from screenshots does have some advertisements included.
This image taken from this great review!
Twittme also offers a native ability to split postings that exceed 140 characters, up to 240 (which begs what do they do with the extra 100). It will then automatically make them into two separate postings.
From testing there is some features lacking, but it does work across devices. This is one to turn to in a pinch.