Here's how this works: You will need to export your Rdio collection in some way. This site will then use the export file to add albums and songs to your Spotify collection.
New! Support for Rdio's native export format. See rdio.com/farewell to generate your export. After you have downloaded your export on the next screen you select the favorites_albumsandsongs.csv file from the 1_favorites folder. This export appears to be the most inclusive and is the recommended format for use with this importer.
There are several other formats that I've tested with this importer.
Once you have your export file, press the login button above to securely login to your Spotify account using OAuth.
On the next screen, you will see a form where you can upload your export file. The importer will then look up all the songs in the file, checking if they exist in Spotify. If a match is found, the song is added to your Spotify collection.
Note: This importer prefers albums over tracks, meaning for each song in your Rdio collection, it will first try to find a matching album in Spotify and, if it is found, this album is added to your Spotify collection. If the album can't be found, it will try to find matching track/artist information and import just the single track. So, if your collection in Rdio consists of a bunch of single tracks, this importer might not work well for you. But the code is on github :-)