I know that lemon, honey, and chamomile all work, but haven't tried them myself personally. Sakina has been trying out honey and cassia, I know. And I believe she's mentioned trying chamomile. I've always been tempted by the chamomile, but like I've mentioned in other posts, I'm not willing to mess with my hair color again. 

Beer and olive oil? I'm not so sure about these two, since a lot of people here oil with evoo and don't see a color change? All I know about beer is that is that its supposed to be a really great conditioner. At least, I've been reading about how great it is over the past few years.

Update: Did a quick check on beer. It looks like most people use it as a shine-enhancing conditioner, and nobody will verify that it highlights or lightens. Method to highlight with beer involves pouring a beer on your head and sitting out in the sun. My opinion is probably any lightening your getting from this is from the sun exposure, not the beer.