What’s your favourite Wordpress plugin?

People love Wordpress because it is so extensible and can be used to power a wide range of websites.

It can be customised with relative ease, and there are more than 4,000 plugins available to enhance the back and front end of Wordpress, adding functionality for administrators and visitors.

There are already a bunch of useful plugin-related lists, which in-the-know authors have compiled, such as 25 useful Wordpress plugins, and the 13 Wordpress plugins you don’t use but probably should, and 20 Wordpress plugins to improve SEO.

But since new plugins are created every day, I thought I’d ask the question: what’s your favourite? Which plugin really does the business for you, or has made the biggest difference to your key metrics, or is helping you to become a much more efficient operator? 

Please let me know in the comments below. A brief description on what it does and why it rocks would be appreciated.

I’m especially keen to hear about the best plugins to add Twitter functionality to a Wordpress site. Having checked a few out I’m wondering whether TweetSuite is the best one to go for, as it seems to cover most of the key requirements. Or should I integrate Tweetmeme, rather than using the TweetSuite ‘retweet this’ buttons? Any thoughts?

[Image by tomsaint11 via Flickr, various rights reserved]