I've written about the subject before, but I particularly like Vernor Vinge's characterization of information technology as the outsourcing of cognition.
The fundamental change that may be taking place -- humans may not be best characterized as the tool-creating animal but as the only animal that has figured out how to outsource its cognition — how to spread its cognitive abilities into the outside world. We’ve been doing that for a little while ten thousand years. Reading and writing is outsourcing of memory. So we have a process going on here, and you can watch to see whether it’s ongoing. So, for instance, in the next ten years, if you notice more and more substitution for using fragments of human cognition in the outside world — if human occupational responsibility becomes more and more automated in areas involving judgment that haven’t yet been automated — then what you’re seeing is rather like a rising tide of this cognitive outsourcing. That would actually be a very powerful symptom.
Google is perhaps the most powerful cognition outsourcing tool to date. I can vaguely remember going to the library twenty years ago, before I had internet access, and spending hours to find the kind of information that is now at my fingertips within seconds thanks to search technology.
Other pieces of software I use to augment my cognitive abilities: Excel, Visual Studio, Gmail, Google RSS Reader, calc.exe, Yahoo Groups, Google Calendar, Firefox, Windows.