YARtLWPaF (Yet Another Reason to Love WordPress and Flickr) - WordPress 1.6 will have a built-in media browser (complete with drag-and-drop addition of photos to posts via the web interface), and the media manager is integrated with Flickr! That's so freaking awesome! I won't have to go to my Flickr account, find the photo, click "All Sizes", and select the correct size, then copy/paste the code to embed it in my post. I'll just click a tab, and drag an image in. Rock. On.
It's already deployed on WordPress.com so you can play with it now if you have an account. I just created a simple post to test it out and it worked pretty well.