Resistance for Mac finds, downloads, and allows you to play music from YouTube. Additionally, it can access your iTunes library and lets you drag and drop the newly downloaded content into it. The software performs well, overall, but may violate the YouTube EULA, so use it at your own risk.
Visually, Resistance for Mac looks much like iTunes 9, having a similar LED display, a library sidebar, and playback controls. In the sidebar you'll find the Resistance music folder, the iTunes library, and two sites, YouTube and Soundcat. In our tests we were able to connect only to YouTube. Searching for a song gives you a list of results with a "Download" button next to each one. Unfortunately, there is no unified search within the app and no option to just stream music. After the download is complete, the song is automatically added to the Resistance's music folder. From there you can drag and drop it to your iTunes library if you wish, or play it directly in the app. Since it's so focused, Resistance for Mac feels snappier than iTunes.
If you're looking for a straightforward way to expand your local music collection, you should give Resistance for Mac a go. It's functional, fast, practical, and even fun once you get used to it. Keep in mind, though, that while it works well now, its dependence on third-party services means that any notable YouTube site overhaul could break it.