Frodo is a free, portable C64 emulator. It focuses on the exact reproduction of graphical effects possible on the C64, and comes in three "flavors" (Frodo, Frodo PC, and Frodo SC) with increasingly higher quality of emulation, but at the cost of higher processing power requirements. Frodo has been ported to a number of systems, including handheld computers. The Unix version can use either SDL, X11, or SVGAlib to display the graphics.
gRustibus is a M.A.M.E. frontend for the GNOME environment. Its goal is to be feature-rich and easy to use. It shows screenshots, flyers, cabinet photos, and info for your M.A.M.E. ROMs, and allows you to configure each game individually. Most operations can be done using the keyboard.
KStella is a KDE frontend to the Atari 2600 emulator, Stella. It currently supports the display of snapshots, labels, manuals, the configuration of individual game settings and of course, the ability to play games with just a click of the mouse. All these features are fully customizable to allow maximum flexibility.