Powertweak-Linux is a tool to tune your system to its optimal performance settings. It started life as the replacement for the now obsolete kernel option 'tune PCI bridges', and has been extended to provide more features and support more chipsets. It can also tune network/filesystem performance using /proc/sys entries, disk elevator settings, and x86 CPU registers (needs 2.2.18 or 2.4.0test kernel), CDROM speeds, hdparm type features, and Sony VAIO backlight. It has both a GTK GUI and a text-based lister.
YanC (Yet another nV Configurator) is a small tool which you can use to change the nVidia and ATI-specific settings (almost) as easily as with the Windows drivers. You can edit the settings for AGP support, the Cursor Shadow, the nVidia logo, Digital Vibrance Control, and, of course, TwinView for the nVidia drivers. For the ATI drivers, it offers support for the overlay settings, anti-aliasing, and dual monitor configuration.