The tappity noise.
One benefit of touch interfaces: the subtle variations of keyboard layouts. Used to be that if you wanted a bespoke interface for your product, you would have to make it in hardware. One of the more half-assed examples: putting stickers on the keys of your keyboard, to indicate some sort of crazy remapping. Folks who went long with the concept ended up with all sorts of crazy stuff, though, like fake gas pedals that controlled driving simulators. Either way, such hardware tweaks affect the entire platform, not just one program that you use on it.
Now, of course, the screen is the interface for so many apps, so you can get away with some more interesting one-offs, such as Wolfram Alpha’s complex keyboard that takes up the entire screen:

In fact, Wolfram Alpha has such a large character set that they provide a unique control, at the bottom left of the custom half of the keyboard, that shows pagination for Greek and astronomical variables:


On the iPad, there’s the row of keys above the standard keyboard in iA Writer:

More subtly, Echofon swaps the Return key for @ and #:

Taking away keys always provides a small risk that users will hurt for what was lost. Echofon’s team appears to know this, because selecting “123” at bottom left switches the keys back to Return:

I haven’t seen a whole lot of people dwelling on the flexibility and freedom that software keyboards can afford us now, but I think it’s really important, both from designers’ standpoints (any such change is dramatic, upsetting the norms of the platform, and shouldn’t be taken lightly) and users’ (affording more contextually appropriate input, with fewer cumbersome kludges to get there).
What other customizations have you seen towards this end?