Fraser Speirs' FlickrExport plug-ins for iPhoto and Aperture are the undisputed kings of bridging the gap between Flickr and your Mac-managed photos. The plug-ins are pretty powerful, offering a lot of strong integration with Flickr by adding photos to sets or creating new sets on the fly, as well as keeping in step with the tags you use in your account. The Aperture plug-in can even keep track of the photos you upload to Flickr and replace them with updated versions - instead of creating a duplicate. Apparently not one to sit in one place for too long, however, Speirs is already hard at work on a new version, FlickrExport 3.
Not many details are known just yet, but Speirs has been tweeting some teasers about the new version, and just this morning published a screencast (.MOV link) demoing some of the impressive new Groups features in FlickrExport 3 for Aperture. Most notably, users will no longer be limited with adding a picture to one group at a time - the new version features a table of all the groups to which you belong, allowing you to check more than one group for adding the images to. FlickrExport 3 will even check with Flickr to make sure you haven't overstepped the upload quota for each group.
[Update: Mr. Speirs stopped by to comment that the new features in this particular demo will also be present in FlickrExport 3 for iPhoto as well; he just happened to be demoing them in Aperture for this screencast.]
No release date has been set for FlickrExport 3 just yet, and I assume the prices will remain the same for the iPhoto plug-in (~ $23 USD) and Aperture plug-in (~ $27 USD).