Free stuff sites make up a very decent percentage of all sites on internet. You can get anything from free screensavers to free software to free iPods to free vacations on internet. Obviously other than a very few select products none of them are 'truly' free. They are not even a 'shareware kind' of product. Most of these schemes will expect a certain action to be performed to claim 'free stuff'.
Normally, signing up to an offer from an advertiser, referring friends to a site or service or buying a related product all are examples of 'actions' expected. For example, free iPod offer that took over internet by craze works on premise that each person will refer 5 of their friends to scheme as well as sign up to a few sponsor offers himself.
What is surprising is that out of all these offers, there are very few that are webmaster-centric. You hardly find offers for free domains, free webhosting or free traffic when you sign up for an offer. This is probably because cost of these services or products are too low to warrant much interest. It can also be that webmasters prefer more direct forms of rewards like cash.
It is not that there are absolutely no free domain offers on net. There are a few but their 'action' expected is either to buy hosting or to sign up and post at forums. The first scenario is a little scary because most web hosting companies will register 'free' domain in their own name. So if want to change web host, you lose domain name and with it all hard work you put in to develop brand image. That leaves posting at forums option. Well, if you asked me to post, say 50 messages and all of reasonable length (atleast 10 - 15 words), it will take me a month. This assumes that I also have something better to do! I cannot wait that long for a domain even if it is free.