Dolphin is a file manager for KDE focusing on usability. Has three view modes: Icon, Detail and it used to have Column View like in Finder (it is not available in current version due to problems with maintaibility). Supports FTP, SCP and other protocols. Highly configurable.
Dolphin supports a dual-pane view but also file browsing in tabs. It has a filter tab that allows to filter large amounts of files quickly according to an expression (e.g. *.pdf would only display the *-pdf-files in the directory displayed). More Info »
Archives of all sorts (e.g. zip, rar, gzip, 7zip) can be displayed as folders. Quick rename operations are possible with tight integration with KRename via an "Actions" context menu.
Preview mode for many filetypes available.
It can easily be extended with different service support for YouTube, DropBox, tumblr et. al.