Either in a good or bad way, albums that you either never intended to listen to and ended up doing so, or tried for whatever reason, and which either reinforced or changed your view of the artiste and/or their music.
I'd have to say that
was a big surprise to me. Expected to hate it, to sneer what's all the fuss, but ended up really loving it. After I had resisted long enough and gave in, and then wondered why I had waited so long...