Tweaks That Don't Work

This may be the subject of some discord, but a great community service would be to help others not waste money on snake oil.

Please let us know what you've tried that had no impact - or negative impact, even - on your system. Also, if there are tweaks or products that are only useful on some systems for whatever reason, please let us know what kinds of systems do and don't benefit from the particular products.

Some of you out there have tremendous experience trying to squeeze every last ounce of performance out of your equipment and probably have a laundry list for us - let's hear it!