Below is a categorization of possible useful Wikipedia bots. None of these possibilities are necessarily recommended or implemented.
Examples of actual bots used on Wikipedia can be found at history of Wikipedia bots.
Some bots are vandals, and they are called vandalbots. Learn more about them at Wikipedia:Vandalism and m:Vandalbot.
WikiProject tagging bots add WikiProject banner templates to talk pages on behalf of WikiProjects based on a set of instructions (for example, a list of categories).
Automatic assessment bots also tag specific project banners with the appropriate class where possible (i.e. stub, FA, FL, etc.). When visiting the talk page of an article that has been auto-assessed, you should remove the "|auto=???" parameter from the template and, if necessary, re-assess the article based on the WikiProject assessment scale.