It's so surprising that Flickr doesn't provide an official tool for downloading images from their website but thanks to the API, Flickr enthusiasts have developed some excellent Flickr Photo downloading tools and our favorite is Downloadr.
Think of Downloadr as an offline browser for Flickr photos. [Get Downloadr Windows only, ~300kb zip]
With Downloadr, you can search, browse and batch download multiple Flickr pictures based on image tag (s), Flickr username, Groups Pools, Flickr user sets or even Interesting Flickr pictures of any particular day.
Images are fetched only from public Flickr photostreams though you have an option to authenticate and download your private Flickr pictures via Downloadr.
The developer homepage is in German but the tool itself has an English interface. More discussion on the Flickr groups. If you are using Flickr, Downloadr is a must have utility.
Related Flickr search tools - FlickrCash, Flickr Leech