Hi @JLA,
The most varied plugins are in the “generic” category; you can find a lot of diverse plugins there. See plugins/generic for these.
A typical way to modify a small piece of a page is to use the TemplateManager hooks to intercept requests for a specific template; see e.g. plugins/generic/googleAnalytics/GoogleAnalyticsPlugin.inc.php for an example of this.
Another way to alter generated output us to use a Smarty postfilter. There are no examples of this included in OJS, but have a look at the Smarty documentation for information on postfilters; your plugin would need to register one of these.
If you can describe a little more specifically what your goal is, I can potentially give you more detailed suggestions.
Regards,
Alec Smecher
Public Knowledge Project Team