Plugins don’t typically have a <head>, please provide more details about what it is you’re talking about, ie. what are you aiming to do and is there a particular plugin involved.
Well …I would like to insert some html <meta> tags into the html <head> section from a plugin.
The plugin is parsing the content for videos, and I would like to insert the proper Open Graph <meta> tags, so that Facebook can discover embeded video.
/Andy
If i assume you want to output content into head section for front facing pages of your site, then what you’re looking for are actions and filters.
http://codex.ww.wp.xz.cn/Plugin_API
http://codex.ww.wp.xz.cn/Plugin_API/Action_Reference
http://codex.ww.wp.xz.cn/Plugin_API/Filter_Reference
See the info on adding actions, you’ll specifically want to hook an ation onto wp_meta to output additional meta tags.