What was your best notebook upgrade?

Simple question, what was the best overall upgrade to your current notebook or any notebook you have owned in general? There is no right or wrong answer to this.
I just want to get an idea of what products have significantly improved the look/feel/operation of your notebook over the years.

I will go first.
The best upgrade to my T60 ThinkPad was when I transitioned between Windows XP and Vista.
the user experience had such a drastic change that it felt like the system was finally propelled into the modern age.
The shift from Vista to Win 7 was similar, but nowhere near as dramatic.
At the time of the upgrade I already had a fast processor, 2GB of RAM, and a 7200RPM hard drive so nothing slowed down and overall my computer felt much snappier.

I suppose another best upgrade for me was also software wayyy back in the day.
I still remember the time I upgraded my ancient Toshiba Satellite from Win 3.1 to Windows 95.
now talk about a huge leap into the future that was!