Google Fonts - Making the web more beautiful, fast, and open through great typography
Font Viewer - Font Viewer is an application which does exactly that - it views fonts!
Typewolf - Typewolf helps designers choose the perfect font combination for their next design project—features web fonts in the wild, font recommendations and learning resources.
NexusFont - Choose and manage your fonts with Nexus Font.
FontJoy - Get smart font pairings in one click
Font Manager - Simple font management for GTK+ desktop environments