If you're a Mac user who uses iPhoto and Yahoo's Flickr photo sharing service then you'll want to check out FlickrExport by Connected Flow. It's a free plugin for iPhoto that adds the ability to export photos from iPhoto to Flickr with almost no fuss. I've been using it since almost the first day I joined Flickr and I love it. It's not without it's bugs, sometimes it will stop uploading halfway through an upload but more recent versions seem to have fixed this problem.
With FlickrExport you can easily rename, resize, edit tags, decriptions and add albums to the photos you want to upload. It also advises you on how much of your monthly upload bandwidth has been used (20MB for free Flickr accounts and 2GB pro accounts).
So if you use iPhoto and Flickr be sure to get FlickrExport. If you use iPhoto but don't use Flickr then I recommend you give it a try, it's free and it really makes sharing pictures so simple.