A barbarian can fly into a rage only once per encounter.
(Barbarian class description → Rage feature description)
This line is making me sad; I want to be able to end a rage and start a new one more frequently than that. Is there anything that will let me do that? Feat, magic item, class feature, racial feature, a template, whatever, anything that gets around that limitation is of interest—though I sorely doubt there will be anything.
Any Wizards of the Coast-published, Dragon, or Dungeon D&D 3.5e material is acceptable. Third-party materials and homebrew are not.