Sandboxie: A very Elegant piece of software engineering.

[SandFly]

In the current age, when malicious, and would-be malicious, black hat's and script kiddies are buying their tools by 'Save to Basket, Pay'n'GO sites, Sandboxie is outstanding in defining the sandbox concept under user control. I've had Sandbox capability by my Internet Security suite (Firewall/AV/HIPS/Sandbox), but there is no fine granular control of the sandbox and I don't trust it. Sandboxie allows the possibility to research and explore the behavior of unknown/potentially harmful software in a controlled environment and incrementally control the lock down of Web Browsers etc.
I've been using Sandboxie for a while now, and have finally coughed up for the paid/registered version - worth every Euro.