eMule is a perfect example of this. This file-sharing program lets you search for, download, and share files with others. It's exceedingly popular, and so there are a vast number of files available to you. In fact, it's the most downloaded file of all time from the Open Source site SourceForge.net --- more than 332 million downloads and counting.
It has all the features you'd expect in a file-sharing client, including some very nice logging and reporting. Note that some firewall programs, such as Windows Vista's firewall, will block this program from making an Internet connection, so make sure to configure your firewall to give it access.