What nutrients are concentrated in citrus peels (not juice)? I just bought my first juicer today and I'm having a blast experimenting with the different favors.

I'm looking at all these juice recipes and they all say to peel the rind off of the citrus fruits when juicing as they leave a very sour flavor. I don't mind extra sour things, so my question is, what nutrients am I gaining if I keep the rind on when juicing the whole fruit (eg oranges, lemons, limes, etc)?

Is it still better to leave the rind off when juicing?
You can look up nutritional information at the USDA nutrient database. Their entry for orange peel says that 100g of orange peel contains: 97 Calories, various minerals, and a few vitamins. You can compare it raw oranges to see how it differs from the rest of the fruit. You can also look up orange juice, etc.

You can also get some of that information (plus comparison to USDA recommended daily amounts and to other foods) at Wolfram Alpha. You can even have it compare orange peel and orange.

As rumtscho mentions, the pith (white part of the peel) is bitter. The zest (outermost colored part) is very fragrant (and flavorful).

Oranges vary in sourness depending on variety, cultivar, growing conditions, etc.