Regardless of what the app does and whether the thing that does is particularly useful, powerful or important for what you need to do (or even well implemented), what is a command-line interface that you had a particularly good experience both learning and working with?

In other words, I'm thinking about command line interface design patterns that tend to correlate with good user experience.

"Good user experience" being vague, what I mean is, including (but not limited to)

• discoverability--learning what features are available),
• usability--those features actually being useful,
• and expressiveness--being able to do more with less words without losing clarity,

but if there's a CLI that has none of those but you still like it, I'd be happy to hear about it.

Edit: Trying to stress more that this post is not about the functionality behind the tool. Looks like most of first responders missed the nuance: whether app x is better than app y because it does x1 ad x2 differently or better does not matter; I'm purely interested in how the command line interface is designed (short/long flags, sub-commands, verbs, nouns, output behaviors)..