If you've ever wanted to save every image from a Tumblr blog, you'll know the official way is painful: open each post, right-click, save, repeat a few hundred times. TumblTwo fixes that.

It started life as TumblOne and was later forked and extended by Johannes Meyer into TumblTwo, which is the version worth using now. The workflow is simple: paste in the blog URLs you care about, add them to a queue, and hit start. The app crawls each blog and pulls down every image it can find.
A few features that make it worth keeping around:

It's a Windows tool, available from the TumblTwo GitHub releases page. Mac users have fewer options here. Downloader for Tumblr on the App Store covers the basics if you just need single-blog downloads.
Worth noting since this tool has been around a while: Tumblr's API and layout have changed more than once over the years, so if a blog fails to crawl, check the GitHub issues first. Someone usually has a workaround already posted.