Er...terrifying, forceful posts aside...

Iwama is supposed to be the style practiced by The Founder at the end of his life; this is the man who created that school of aikido:

http://www.youtube.com/watch?v=21y32ASY3Sw

I'm astounded by his power. His son is currently the head of the style, and I find him to be very similar to his father.

Tomiki aikido, as said above, contains competitions; it is very pedagogical - as is Yoshinkan style.

Tomiki style: http://www.youtube.com/watch?v=gHAaK0jipIY

Yoshinkan is very martial orientated, and based on the 1930s ethos, when Japan was engaged in war with China; it is taught to the Tokyo riot police:

http://www.youtube.com/watch?v=vG9y9rqRo9U

http://www.youtube.com/watch?v=AIt_g0amnis

Aikikai is the mainstream style; it is very diverse, and indeed: the Iwama style is aikikai. I see it as being a middle ground between too martial, and too soft (i.e., obsessed with 'ki').
Some aikikai teachers:

http://www.youtube.com/watch?v=s2mFrh3FXQ4

http://www.youtube.com/watch?v=3iy9sIDfN0Y

http://www.youtube.com/watch?v=3yiBehK4vBU

http://www.youtube.com/watch?v=dHnT1hUNmTo

...and the head of the Aikikai:

http://www.youtube.com/watch?v=94sOPZ4WtyY

Hope that helps.