I don’t know of any particular widget you could use for this purpose, aside from social bookmarking icons one places at the end of each post. http://wordpress.com/forums/topic.php?id=4714&replies=34
You could also make your own unique text widget simply stating that you enjoy visitors and commentary and would like your readers to recommend your blog to other bloggers.
There is no poll widget as yet for wordpress.com but you can have a poll through an off-line provider. Because you’re new I’d like to offer you something that will help you locate information. This will introduce you to some basic wordpress resources.