Apps similar to Blue Crab include Wget, which is free and open source. Other options are cURL, uGet, SiteSucker and WebCopier. In total there are five Download Managers like Blue Crab for Mac, Windows, Linux, Android and iPhone.
GNU Wget is a free software package for retrieving files using HTTP(S) and FTP, the most widely-used Internet protocols. It is a non-interactive command line tool, so it may easily be called from scripts, cron jobs, terminals without X-Windows support, etc.
United States
cURL is a computer software project providing a library and command-line tool for transferring data using various protocols with URL syntax: supporting FTP, FTPS, SCP, SFTP, HTTP, HTTPS, TFTP, TELNET, DICT, LDAP, LDAPS and FILE.
Sweden
EU
uGet is a lightweight and full-featured Download Manager for Linux and Windows. uGet allows you to download in multiple parallel streams for download acceleration, put files in a Download Queue, Pause & Resume downloads, Advanced Category Management, Browser Integration...
Thailand
macOS application that automatically downloads websites from the Internet. It does this by asynchronously copying the site's webpages, images, PDFs, style sheets, and other files to your local hard drive, duplicating the site's directory structure.
United States
WebCopier is an easy-to-use website mirror utility. It allows to download entire Web sites, store them locally and view them later, at users' convenience. WebCopier provides access to the downloaded information at any time, no matter where its users are, no Internet...
United States
