Look similiar to LimeWire, but it’s totally free and has many powerful features to find and get what you want : media, software, collection and much more … For users who always search for files to download but can’t find it on Google or just have habit to collect rare stuff, I think FrostWire, Amule and an Bit Torrent client like Azureus or Utorrent are suitable for you. 
Download and learn more at official website : http://www.frostwire.com
After download, you open the .deb file and click Install Packages to install. To start FrostWire, go to Menu - Applications - Internet - FrostWire
For first starting, it may load very slowly but be patient, after the first initialization it will open faster.
It’s also available for other OS beside Ubuntu like Windows, MacOS, Red Hat … see Other System
