TumblOne is a free program developed by Helena Carver.

It allows you to download every image from a Tumblr blog of your choice. This is useful for blogs that are deleted often that have images you want to backup to your hard drive.

Update: There is a new, updated version called TumblTwo.

TumblOne screenshot

History of versions

TumblOne 1.04 - download as exe or zip (latest version)
TumblOne 1.02 - download as exe or zip

TumblOne SourceForge mirror.