I like the avatars used on this site (drupal.stackexchange.com). They are "squares with some colors". Are they available and are they for free?
Those are generated by Gravatar based on your email address and some salt.
You can e.g. follow this link: http://www.gravatar.com/avatar/5e17e2cc-3e32-473a-9e8c-20bdca6c138e?s=32&d=identicon&r=PG and generate the uuid part in a unique way for each user