Google SERPs Experiment: Top Ranking Site Drill Down
Lenssen points to New Google Test: More From The Top Result, which shows info parsed out of the top search result for the term "Nescio."

Lenssen calls this a "categories" experiment, but it looks from these two screenshots of non-english searches:
Screenshot 1
Screenshot 2

...that Google's pulling out site pages it thinks are particularly relevant so searchers can more easily find info from that top ranking site. The labels of the links - from these two examples - seem to come from the url of the page itself.

I find it interesting that in this experiement Google drills DOWN into a single site from the SERPs (rather than enabling more of a conceptual drill down), and into that the first-placed site at that.

I suspect they're targeting folks who're used to clicking on the first link, perhaps to encourage them to learn a little more about the site, or to make it easier for them to drill deeper into the site without visiting.

A sort of site preview thing?

If this is something that becomes widespread your page naming conventions had better make sense and be accurate ;)

Also note the Green Bar SERP experiment (also from Lenssen).

This top ranking site nav is not listed in Google's SERPs test blog post (green bars is).

Update:
This is distinct from Expandable Search Results

And it seems to be distinct from the Google Categories as Barry's been digging into them on SEW:
Google Web Site Categories Explored
Google Testing New User Interface For Web Categories?
Garrett French - MarketSmart Interactive | permalink | +marktd |