Wossis.com uses a whitelist to control which sites may be linked to in user-submitted content. It's an attempt to limit spam - unwanted advertising.
Some of the entries on the whitelist (such as wikipedia, wiktionary, gimp.org, inkscape.org) are added out of gratitude to those sites for their own activities. If there's sufficient demand to add a site with genuine altruistic or philanthropic (but certainly not 'how to get rich' or 'improve your love life'), it'll be added for free to the whitelist.
If - for example - a company wanted to promote its products by making a set of flashcards and link them back to its catalogue, then you can also add your site to the whitelist. There's currently no automatic way of doing this - it's on the (extensive) TODO list. You could try nominating an URL for addition to the whitelist when you make a donation to wossis.com.
Whitelist nitty-gritty.
It's a list of hosts, so for example the site for the Model railway article at Wikipedia is en.wikipedia.org - the English version of wikipedia. If the site for wikipedia in a different language (an omission, tell us about it) is missing, you will not be add links to it in your comments.