There are my three most-used WordPress plugins:

Postalicious (Get this plugin)

Postalicious is a WordPress plugin that automatically posts your delicious, ma.gnolia, Google Reader, Reddit or Yahoo Pipes bookmarks to your blog. The exact details of how your bookmarks are posted are very customizable and are designed to meet your specific needs.

I use this plugin to create my Great Reads posts – it lets me find blog posts, stories, news and anything else online that I think is interesting to the readers of this blog and tag it with Delicious, adding my notes about the link in the field in the Delicious bookmark.  It checks for my saved items, and once I have five things it posts it for me!  I really love it

Twitoaster (Get this plugin)

Automatically retrieve Twitter Replies and/or Twitter Retweets to your Blog’s Posts. These Twitter mentions are handled like Posts Comments, using WP built-in comment system.

I like this tool because it brings in conversation from Twitter to the comment stream for the relating posts.  What I wish was improved was the tracking to pull in the full conversation, and not just retweets, etc.

Tweetmeme (Get this plugin)

The TweetMeme retweet button easily allows your blog to be retweeted. The button also provides a current count of how many times your story has been retweeted throughout twitter.

I like this plugin because it puts an easy-to use button at the top of posts for people to tweet out a link to the post if they want to share – it even pre-writes the tweet for you with the link!

What are your favorite add-ons, extensions, or plugins for blogs?