Nusach Ashkenaz: the most common in America. There are variations, but Artscroll's Ashkenazi siddurim are very widely used.
Nusach Sephardi: There are actually many variations according to the particular Sephardi community. There is no authoritative Sephardi nusach like the Ashkenazim have.
Nusach Ari: (You may hear this called Nusach Sfard.) Nusach Ari is a version of Nusach Sfard (also spelled Sefard) used by Chabad chassidim. If someone calls this Nusach Sfard, don't make the mistake of thinking it is Sephardi. Nusach Sfard and Nusach Ari are very similar to Nusach Ashkenazi but attempt to include kabbalistic ideas and customs. Each chassidic group has its own version of Nusach Sfard, but the Ari is most well-known because of the wide geographic reach of Chabad.
If you want a cool Jewish hobby/hoarding opportunity, consider collecting siddurim!