Opinion polls can be fun and increase interactivity between you and your readers. They can also help folks come back often. We've got two we've been using:
Wp-polls, using an AJAX poll system: http://wordpress.org/extend/plugins/wp-polls/
SodaHead Polls ( frankly the name is just fun!) http://wordpress.org/extend/plugins/sodahead-polls/
Here's the link to my search query at WP site so you can browse their site for more options: http://wordpress.org/search/polls