I use:
Smart Update Pinger & Jerome's Keywords (similare to Walrus' mention of Tag Warrior, I think) both available from
here
MyLinkOrder - Easily change the order of links and link categories on your sidebar rather than the way WP tells you to have them.
Post Updated - add a "Updated on..." after the "Posted on..." date for updated posts.
Redirect Old Slugs - Could be useful as your a beginner and might be changing things next week
SpeedyCat - Quickly add new categories while writing a post.
vPIP - For putting video into your posts
and last but not least (for now anyway)
Wordpress Heat Map - for putting a category 'cloud' on your blog
I have a few others (some homemade), but these are used the most often