There are several other free modules that ExpressionEngine has which have not been covered here.
The Discussion Forum is a module written by EllisLab but sold separately from ExpressionEngine (although it requires ExpressionEngine to run). The module leverages ExpressionEngine's Member module features, but has its own templating system and is styled via separate CSS. The best example of a discussion forum in action is the ExpressionEngine support forums (at http://expressionengine.com/forums).
Simple Commerce allows you to mark posts in channel entries as being for sale. You can create a channel of items for sale, set the price for each item and then accept payments through PayPal. All payment information is transferred directly between the visitor and PayPal — at no point does your website or ExpressionEngine handle payment information. Although Simple Commerce is not a fully-fledged e-commerce module, for smaller websites that do not need the complexity...