Sometimes, you want to delete a whole bunch of users, terms, or nodes, say to cleanup a site while developing and still keep its configuration, so reinstalling is not really an option, and the normal content- or user-administration pages get in your way because they only show a limited number of entries, meaning you have to delete page after page of entries.
Of course, you could just use node_delete or user_delete in D6, user_cancel in D7, and loop on all values. But there is a simpler way: just install devel module, and enable its devel_generate submodule.
The main purpose of devel_generate, as its name indicates, is to create content, be it nodes, terms or users. However, much like you use the Start button on Windows to Stop the OS, you can use devel generate to delete content instead of creating it, by enabling its optional Delete all content in these node types before generating new content checkbox. If you then choose to generate 0 entries, you'll get just a nice cleanup, without content generation, and nothing to write.
And for you Drush users, there's even an additional bonus: devel_generate has a Drush plugin implementing three equivalent commands: drush generate users|content|taxonomy. These three commands take as parameter the number of entries to generate, plus a "kill" option, which is here to remove existing content in the same way the checkbox specifies in the UI.
Brilliant!
Not only can it get rid of all of the content, but it can do it by node type as well. That's genius.