For me it is most software that works well with a keyboard. Examples being:
Opera - fully configurable with keyboard shortcuts - no mousing required.
Spotify - easy to tab and type music - no mousing required
Picasa - arrow swipe gestures between pictures work well.
Windows XP (I find Windows 7 more cumbersome for keyboard use ie no keyboard favorites etc)
Anyone else found that a particular piece of software works like a dream with their device?