uGet is an open source download manager which uses aria2 and curl under the hood, available for Linux, BSD, Android and Windows.

flareGet, a download manager for Linux that supports dynamic file segmentation, HTTP-pipelining for accelerated downloads and more, has been updated recently, receiving quite a few improvements and bug fixes.

The application is written in Qt and besides the icons, it integrates pretty well under GNOME, including desktop notifications and an AppIndicator for Unity.

uGet is a GTK3 download manager that can make use of aria2 (a command line download accelerator also used by apt-fast) to accelerate downloads, and comes with features such as: clipboard monitoring, scheduler, resume downloads, batch downloads, supports multiple protocols (HTTP, HTTPS, FTP, BitTorrent or Metalink - these last 2 require the aria2 plugin), HTML and TXT file importing, proxy support, cookie and user agent control and much more. 

Other uGet features include: scheduler, clipboard monitoring, can be integrated into Firefox through Flashgot plugin, can import downloads from HTML files, supports batch downloads and more. 

JDownloader is a free, open source download manager which makes it easy to download files from one-click hosting sites (supports more than 110 services) including using premium accounts, support for downloading from YouTube, Vimeo and other such websites, it allows parallel downloads, automatic CAPTCHAs, automatic archive extraction, automatically acquire a new IP (if your router is supported) and more.

JDownloader 2 beta is available for download. The new version brings a reworked user interface, simplified settings, new options for some of the available plugins and more. The JDownloader Google+ page also says there are "many new features and improvements under the hood" but unfortunately, there's no official changelog for the latest JDownloader 2.

Among its most important features are captcha recognition, automatic archive extraction and reconnect (useful for most file sharing websites: pyLoad disconnects your Internet connection and then reconnects so you get a new IP so you can continue downloading, bypassing limits imposed by various websites).

JDownloader is a cross-platform download tool which supports most popular file sharing websites and comes with captcha recognition, multi-part downloads, tweak wait times, auto-reconnect, scheduler, automatic file extraction and more.

To integrate JDownloader with Unity, you can use a package called Unity-JDownloader which adds quicklists, a speed counter (Unity badge), progress bars as well as notifications for completed downloads.