GeekShadow
Awesome ! I didn't test it yet, but it should be better and more useful than my addon :)

Sofy is for the Songbird users who also use Spotify. Sofy will attempt to match all the tracks in a playlist to the corresponding tracks in Spotify. Only exact matches will be found, i.e same album, artist and track name as in the metadata. Links to the matching tracks in Spotify are embedded as metadata in the track in Songbird. When all tracks have been checked, a list is presented with links to the corresponding Spotify tracks, artists and albums where available. This search only needs to be done once.
Also, the tracks checked will be placed in one of two new playlists called Not on Spotify and On Spotify.
This add-on was developed in collaboration with SAPOB
Awesome ! I didn't test it yet, but it should be better and more useful than my addon :)
Now if only it could launch the tracks by itself without having to click on them individually we might be on our way to some kind of integration of Spotify within Songbird. Of course, that might require some advanced coding. Something I don't have the first clue about. Like I guess it would have to take the track length to know when to launch the next track, etc.
Not working to well for me. :/ When I click a track with the Spotify symbol it launches songs by completely different artists and sometimes the right artist but the wrong track. I'm very neat and obsessed with my tags so they are all right (due Spotify's seldom in my case are) so I don't believe it's something wrong with my tags.
Other than that, great idea!
I'd really love to try it,but i'm using Songbird 1.7
Some help?
Here is a version that should work :
http://wiki.geekshadow.com/doku.php?id=add-ons
andreas.f
If the search gets stuck on a track for more than 1 minute, press pause, wait a moment, then resume the search.