Firstly , PR10 is considered the highest pagerank of a website by google based on the quality, contents, web presence, backlinks, traffics etc of the website. There are a handful of PR10 websites on the net including google, yahoo, facebook, tweeter to name a few.
Apart from these highly popular sites there are still some other Pr-10-directory.com sites such as gramrox and zoom h2n based on the directory category and of course some online forum sites are given PR10 rank as well.
Search engines give more importance to PR10 directory sites and of course if your website is associated with Pr-10-directory.com sites (in terms of reciprocal links) then your website has more potential to grow and become popular in the long run.
This is only my point of view, I just thought I should share with everyone.
Thank you.
NKGurung
Webmaster