The basic install of PD is referred to as Vanilla. Although many things are possible with this plain version, some additional libraries are very helpful and there is a handful which can be considered standard.
Find and Install Extensions
PD comes with Deken, a builtin tool for installing external libraries. Deken can be opened from the menu of the PD GUI. On Linux installs it is located under Help->Find Externals. Deken lets you search for externals by name. The best match is usually found at the top of the results. cyclone is an example for a library with many useful objects:
Deken lets you select where to install externals in its Preferences menu. Everything will be located in the specified directory afterwards.
Add Libraries to Search Paths
Once installed, it may be necessary to add the individual libraries to the search paths. This is done in an extra step. On Linux installs, this can be found under Edit->Preferences->Path: