Well, in actuality there is not really such a thing as a free web host. There may be hosts out there who offer a limited set of services for free. However, you will be hard pressed to find one that will offer full service without either upselling you part of the full services, or who do not tack on mandatory advertisements on your pages.
Advertising
A majority of free web hosting companies generate a large portion of
their revenues through advertising. They may require you to put a
banner ad on your website, or require that your site launch pop-up
advertisements each time users come to your site. A growing number of
Internet users find this advertising annoying. Pop-up and banner
advertisements may make your site appear unprofessional to some users
(not all though as pops have gone mainstream). Users can often
recognize the free web hosting advertising based on some types of
advertising (the old geocities was an easy mark).
Upsell!
The services that you get from a free web hosting company are
typically very basic and limited in nature with regards to things like
reporting, database access, and static IPs. Adding additional services
will always cost you money, and you may soon find that after you have
added a few upsells that the "cost" of your free web host outruns the
cost of a shared web hosting account.
Domain Registration
A few free web hosting companies require that your domains be
registered by them. Doing so may result in an overall higher
annual registration fee, quite possibly they may charge you an above
market rate domain transfer fee.