There was a time when Linux users didn't have any fancy option when it came to music players. Many of them were still using command-line players, while the options for those using a GUI were restricted to xmms (a very simple mp3 player, resembling WinAmp quite much), mplayer and VLC. There was no way to have one of those fancy music players that were already common on proprietary platforms, like iTunes or Windows Media Player.
Today, for the great joy of music lovers, things are different and Ubuntu offers a wide range of options when it comes to music players. gMusicBrowser is a modern, fancy jukebox, designed for those who have to handle large music libraries. gmusicbrowser supports the most popular formats for digital music, like mp3, ogg and flac, it offers to the user the possibility to customize layout and interface and it integrates a music browser detached from the player, so that you can browse your collection without messing up with the file you are playing and enjoying. gmusicbrowser includes complex filters, with unlimited nesting of conditions, the possibility to retag single or multiple songs and a tray icon, which can be used to manage the player, stop, play and move backward and forward.
For those who can afford it, there are many valid alternatives to gmusicbrowser: Banshee and Rhythmbox offer more functionalities and options than gmusicbrowser but for sure they require a bit more in terms of resources. If having a fast music browser and player on an old machine is what you need, then gmusicbrowser is definitely the answer to your problems.
gmusicbrowser comes with a GPL license, so that it is possible, for those able to code in perl, to modify and redistribute it.
gmusicbrowser can be installed from the command line, adding PPA repositories with "sudo add-apt-repository ppa:shimmerproject/ppa" and then typing "sudo apt-get update" and "sudo apt-get install gmusicbrowser". The website of the developer offers a .deb package which can be installed with "dpkg -i gmusicbrowser_1.0.2_all.deb". The source code is also available for download, for those who want to compile the software themselves.