Well, are you thinking lemon (lemon curd, orange curd..) raspberry, etc..... It is so easy to mix fruit purees, curd etc into buttercream, just mix them in at the end. They are usually pretty stable, expecially if you can chill them/cake before hand a bit. An excellent reference guide (library, amazon, local book store) is "The Cake Bible" ... it's such a must have book 
Yep cake bible is the best! amazing recipes. Fruit purees are more work but some extracts can be bitter when used in big amounts. Raspberry buttercream made with fresh raspberry puree is awesome a favorite of my clients. Lime made with lime oil instead of an extract is amazing too. The author of the Cake Bible is Rose Levy Barenbaum (sp of last name may not be correct) she has a lot of her recipes on line also has her own site with some of her recipes.hth
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