I'm only 3 years behind....
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjbbSmSNYMXkgv7Evhd-qoL16jL9hroalwi5Xy8pFWOUYEyPCW0qEHcD_rAIjyG00u-2tYDPICExEtnZShgGC5ghhUBTVphTdzLxf7OAjHYbSUYm5jevQD4dA6VLW2cCHBv0kALRAGTrlQ/s320/portal-icons.jpg)
It's really cool, using a first-person-shooter interface to solve a series of puzzles. It turns out that the independently created flash version of the game was a really good primer.
It was always cheap, but now, it's FREE. Highly recommended, assuming you didn't play it already.