As of late, I've been experimenting SharePoint v3's Feature Feature for adding new functionality and UI elements to a SharePoint v3 site. So far, I've just been playing around with the meager examples in the SDK and trying to fill in the gaps in the documentation by looking at the out of the box feature definitions. One learning that I did have is that while experimenting with Features, IISReset must be your friend. It seems like all the xml definition files are cached in IIS's memory so changing them on disk has no immediate effect. Also surprising is that going thru the cycle of deactiving the feature, uninstalling the feature, making the change, installing the feature, then activating the feature had no effect. It still used the version that was cached into memory. You must recycle your application pool before the change will go into effect.