Text Editor Customization

How can I customize the text editor in the 'add an item' page so that a richer text can be added?
Thank you in advance,

What specifically are you looking to add? More metadata fields or additional HTML tags?

Currently, you can add item types metadata fields, http://omeka.org/codex/Managing_Item_Types. And you can edit the text in a field by clicking the HTML box for that field and either use the HTML text editor or by adding your own tags.