As with anything Google it’s all part of an algorithm that no one but Google knows. The one common factor is that this typically occurs only for the top ranked site and is usually a major brand/popular site. You can read more about it in Matt Cutt’s blog.
this is something which I am interested on my site to be listed like this in google, but i think will take me a lot of time for that…
I just got notice of a comptitor just announcing their new site, so i decided to check it out. the site it brand new, they have a PR0 but they are getting these sublistings in their google listing? They are a child company of another company that has a very popular website but I don’t see how that should matter?
One of my competitors have these listings, and the website shows again on the second result!
Same as linux
My competitor’s website is 6 years old, ~10000 in alexa, have a keyword rich domain name but isn’t well SE optimized!!!
BTW, thank you for the link, I was looking for it
if you have complex site index google divided into sub section. that’s all
It’s not quite that simple.
Through out my experience , i never think about that, if you have large site 3or 4 levels. that’s all. don’t think too much about that. create as many pages you can.
It’s not a race 
I would say create as many pages as it takes to communicate your information to your visitors effectively.
All it is, google is providing it visitors with other links from your site that is also related to what the person may be looking for and may not be able to find it that easily while browsing your site.
That may be the reason for them, but I’m not 100% sure.
I just logged in to ask this same question, so thanks for saving me the trouble!
I noticed that Sitepoint itself has these sublinks.
I got it in under a year
If this helps any, they are called Sitelinks:
The links shown below some sites in our search results, called Sitelinks, are meant to help users navigate your site. Our systems analyze the link structure of your site to find shortcuts that will save users time and allow them to quickly find the information they’re looking for.
We only show Sitelinks for results when we think they’ll be useful to the user. If the structure of your site doesn’t allow our algorithms to find good Sitelinks, or we don’t think that the Sitelinks for your site are relevant for the user’s query, we won’t show them.
At the moment, Sitelinks are completely automated. We’re always working to improve our Sitelinks algorithms, and we may incorporate webmaster input in the future.
I must admit that I seem to see these 5 or so links for most, if not all of the popular and large well known sites that seem to have some authority or may have some. I don’t seem to see these for the newer or smaller sites, so I have a feeling it also has things to do with the amount of traffic the site receives, authority and also the sites link structure. Just my 2 cents worth, but most probably I am wrong.
I have seen this for smaller sites, also one that I created long ago, it takes time though, and I think it does matter to have a Google friendly site to obtain this, but as Google changes things all the time, there are no easy solutions.
check the new section under “links” in the google webmaster tools.
While it doesnt allow you to add sitelinks, it does give a clearer understanding of how google appropriates them.
befor my backlink=null my PR=2,
Now,my backling reached 12,and also PR=2
The actual PR and the PR shown in Google toolbar differ. The PR in Google toolbar updates in about 3 months.
There is no known conversion rate for backlinks to PR. If you had 100 backlinks, what would be your PR–who know’s what could be the answer.
Also, it’s not only about the number of backlinks. It’s about the quality of backlinks.
I was reading on another community that google creates those site-links if those pages are viewed the most by visitors.
We know that’s not true because Google doesn’t have that kind of information.
I’m lost here? Are you saying that google can not tell how many people have visited a certain page through their search engine?