The best Ska I've heard was from a band called Capdown. I am currently in love with music from Nujabes, who's a DJ type guy who makes the most amazing music. As for reggae, you can't go wrong with all of Bob Marley's music. I recently discovered a band called 311. Their song "Reggae Got Soul" is awesome. As for alternative, I would go for some Big City Rock. They're awesome. My favorite band of all time is Family Force 5. They are AMAZING. Other favs: Jack Johnson, Landon Pigg, The Hush Sound, Just Jack, The Robot Explosion, Senses Fail, Sia, Under the Influence of Giants.. The list goes on. To find artists that are similar to what you specifically love, I would suggest Last FM. Its great for that stuff. I found tons of bands through it. Good luck :]