Though there is an abundance of download managers for Windows, its open-source counterpart doesn't have that many applications to boast of. There are only a few applications in this category; however, unlike Windows, these ones don't come with any spyware or funky toolbars that get installed the moment you install the software. These apps simply do the job.
jDownloader is an open-source, platform independent, download management tool that is written totally in Java. The application offers uninterrupted downloads, Captcha recognition, automatic decompression along with many other features. The application works across all platforms be it Windows, Linux, or Mac and needs only Java Runtime Environment for it to work.
FatRat is an open-source download manager that is written in C++ and Qt. Once installed, the application will let you have uninterrupted HTTP as well as FTP downloads. With support for SOCKS5 and HTTP proxies, the application can let you download any file you want from anywhere. Another great feature about this app that I personally love is that it supports BitTorrent. This means that you can search for your favorite content on popular torrent sites and start downloading right away. What's more, you can also download stuff from RapidShare using the software.
Wget is still my favorite download-management application. Simple and easy to use, this command-like software lets you download anything without any interruptions. With support for HTTP, SFTP, and FTP protocols, the application lets you download most of your files uninterrupted by simply a single command. If you are not a big fan of command-line tools, you can also install the graphical frontend for the tool known as GNU Wget.
Written in C++, MultiGet is an easy to use GUI downloader for Linux. The application, once installed, lets you download files via HTTP, FTP along with support for proxy. One of the most unique features about this application is that it comes with support for P2SP. This means that while downloading, the application will fetch files from multiple locations (servers) and then combine the data into one single file. This, according to the developers, significantly speeds up your downloads.
KGet is KDE's download management application. Once installed, it lets you download files and restart a download whenever it is interrupted. You can see whether a download is current or pending and can download files straight away from Konqueror (KDE's web browser). This integration with Konqueror is what makes this application perfect for KDE users. Furthermore, there is also a metalink support. This means that you can paste multiple links and begin your downloads.
Note: If you're a Firefox user, you can integrate KGet with your browser by installing the Flashgot extension for Firefox. Once installed, you'll be able to download all files in Firefox via KGet.
Tucan Manager is a free file-sharing and download-management application that works across Windows, Mac, and Linux. The open-source tool supports Captcha resolution, downloading from multiple hosts, and premium user accounts. As far as the UI is concerned, the application's interface is quite straightforward and lets you add downloads with a click of a button. Unlike other applications, Tucan is slim and lightweight and can work perfectly across any platform you're on.
Axel is more of a download accelerator rather than a download manager. Almost similar to wget, axel can use multiple mirrors for downloading a single file. This, in turn, significantly, speeds up your downloads. Another great thing about this application is that it has no dependencies whatsoever and is extremely lightweight.