It shouldn't come as a surprise to you that umbraco is open source, and probably not much of a surprise that there's a ton of free packages out there for you. But did you also realize that most of those packages comes with full source code as well?
For most of the packages that I've initially build, I've added them to a codeplex project called codeplex.com/umbracoext:
It includes full source for the Dashboard controls, the MemberControls, the RSS Package, the Utilities and the new umbSearch that indexes Office and Pdf files as well. Casey Neehouse have added his AutoForm Plus package as well, and anyone else is welcome to share their work here.
So if you need to modify a package or are curious how the RSS package are extending the umbraco UI, all the answers are here - for free.
Sharing is Caring - merry Christmas :-)