The package repository is a good start - it contains packages that are approved by the Package Repository Group. You can access the package repository directly in umbraco by right clicking the "Macros" node in the developer section and choose "Import package".
The Package Repository Group is still (by November 2007) working on collecting and getting documentation for the many packages that are spread across blogs and the forum and are expecting that the Repository will be more complete by Q1 2008.