Here on xoops.org, we had a considerable
Module Repository with many modules for many purposes. This module repository has not been updated for some time (For reasons I will not go into here it had to be closed) and as a result modules from here may not work effectively with xoops 2.0.16. However, the repository is still open - hopefully some clever people may pick up one of these archival items and breathe new life into it.
In the mean time Xoops news has a modules category where developers are announcing new releases of modules.
click here to browse this news category or use the 'search' function here at xoops to find what you are looking for.
Alternatively, a Google search of "XOOPS" and "Module" should reveal additional sites with XOOPS Modules available for download. There are also links to reliable xoops developer sites for modules at the bottom of the xoops 2.0.16 (latest release) notes.
Module testing site
If you want to test out some of the common modules from an admin point of view check out:
xoopsmods.mmwebsites.com