The new Retina MacBook Pro has an astounding 2880x1800 pixels. Naturally, the "native" double-detail resolution is 1440x900. But what other resolutions can be used? And how crisp are they? (Are they just up-/down-scaled @2x graphics?)
How does the scaling work?
According to an article in AnandTech:
There is a new option to configure the resolution in the system settings. While you actually change the resolution, Apple only refers to the visual impact of the settings.
So basically changing the resolution offers:
(original picture from AnandTech)
Due to the imprecise definition of "retina display", this answer may seem confusing, because at the top Apple speaks of 2880x1800 as retina, but the picture suggest something different.
This also gives a good understanding on how it works: http://www.slideshare.net/MarkRiggan/retina-display-textures
Another clear answer is obtained from using a Windows OS on a Mac. So, in terms of pure Hardware support (the other answers gave great summaries of software-answers, and I am by no means advocating or bashing Windows-on-Mac solutions, just giving the pure-hardware capabilities of the MBP w/Retina), the resolutions are:
2880x1800 2560x1920 2560x1600 2048x1536 1920x1440 1920x1200 1920x1080 1680x1080 1600x1200 1440x900 1400x1050 1366x768 1280x1024 1280x960 1280x800 1280x768 1280x720 1152x864 1024x768 800x600
From this, it seems the max resolution is 2880x1920, not 2880x1800, but I cannot verify that. It seems like the maximum horizontal resolution is 2880 and the maximum vertical resolution is 1920, but not both at the same time. Further investigation would be nice.