For a general idea of the stylistic differences you can see a lot on youtube, but I agree with Mary and Andrew for where the real differences can be found.
Here's a quick, incomplete sampling of video:
Ki Society
http://www.youtube.com/watch?v=Wc_v_bqq1aA
Yoshinkan
http://www.youtube.com/watch?v=vsZFfFtMSh8
Aikido Kenkyukai
http://www.youtube.com/watch?v=8pqGHLrOlI0