I’m a really big fan of Flickr and have been using it ever since Abby was born.
The biggest pain with Flickr is using its web-based interface. Now don’t get me wrong, their UI is very nice, it’s just that iPhoto is local, fast, and convenient.
FlickrExport solves this problem by providing Flickr integration into iPhoto. Once installed, from within iPhoto you can simply highlight the image(s) you want to upload to Flickr and press CMD – Shift – E. This brings up the export screen:

FlickrExport can set image titles, descriptions, resize your images, set tags, and privacy settings. FlickrExport is shareware and available for 12 British Pounds (about $25 Canadian).