Okay, there's a bit of a journey behind this problem and a sequence of bad decisions and messups that put me in the position I am in right now. Although the biggest purpose of my macbook pro (2011) is for schoolwork, I occasionally play Valve games through Steam such as TF2, Counter Strike, Half Life 2 and Garry's Mod. I was watching a youtube video about TF2 about a month ago and the commentator recommended that if you're having trouble aiming, you could turn off "mouse acceleration" in your computer, and he briefly explained what it was and why it affects your aiming performance. Well as soon as the video ended, I immediately googled how to turn it off, assuming my computer had mouse acceleration which was the reason for my relatively poor aim. As it turns out, mouse acceleration in my computer was ALREADY off, and I just really sucked at aiming. Of course, at the time I didn't know that, and went to this site in my ambition to turn off mouse acceleration which I didn't know was already disabled: http://osxdaily.com/2010/08/25/mouse-acceleration/ If you scroll down the website, and look under the part that reads "How to disable or adjust mouse acceleration" you'll see a command that is given that the article claims will disable your mouse acceleration when entered into Terminal. I entered it, assuming I had nothing to lose, but boy I sure did. After entering the command into Terminal I logged out and back in, and immediately noticed a change in my mouse. It felt like I had less control over it but I was convinced that it was simply the "magic kicking in" until I went on TF2 and realized how badly I screwed up. Not only is it aggrevating to know that I've only worsened the situation, but it also feels incredibly uncomfortable just moving the mouse around when doing schoolwork. So I took my computer to the Apple store and the apple guy showed me to the mouse sensitivity section and told me to simply lower the bar, which should fix my problem. But he clearly didn't understand that mouse acceleration and sensitivity are very different, and mouse acceleration is relative to the distance of where the cursor travels according to the speed, and sensitivity only changes the speed of the actual mouse. So now I'm using my computer and no matter what I do I get this incredibly itchy feeling whenever I'm using the mouse to do anything, and I've looked through countless results in turning it off but all softwares that claim to kill mouse acceleration are not available for Lion (which I'm running on.) I can't imagine how it could be so hard, if it was turned on so easily then why is it seemingly impossible to disable? Does anyone know how to disable this?