Fairly old game I got addicted to recently. It's very original. You play as a hacker hired by a secret company and do odd (usually illegal) jobs for major cooperations. Below is Introversion's description:
"You play an Uplink Agent who makes a living by performing jobs for major corporations. Your tasks involve hacking into rival computer systems, stealing research data, sabotaging other companies, laundering money, erasing evidence, or framing innocent people.
"You use the money you earn to upgrade your computer systems, and to buy new software and tools. As your experience level increases you find more dangerous and profitable missions become available. You can speculate on a fully working stock market (and even influence its outcome). You can modify peoples academic or criminal records. You can divert money from bank transfers into your own accounts. You can even take part in the construction of the most deadly computer virus ever designed."

I'm thinking of buying it, but I'm kinda poor, so I duno...
You can do quite a bit on the free version anyway.
Mac:
http://www.ambrosiasw.com/games/uplink/PC / Linux:
http://www.introversion.co.uk/uplink/2/