In case you are looking for a shopping cart to add to your repertoire, I have heard that the Substruct is really good. Even though, it does not have categories, just tags. When doing blogs or content on site, Wordpress is the best option. It is CMS blogging software that uses php, plug-in, and has the best user friendly interface with capacity for documentation.
vBulletin is the better choice for forum designing and development. It is very powerful and contains an awful lot of plug-in options.