Sandboxie Plus is an OS-level virtualization solution. It creates an immediate isolated environment in which applications can make changes that are gone as soon the sandbox is closed. Originally a limited shareware called just Sandboxie, it was eventually abandoned by its corporation but it did make sure to release its source code to the public, and it was thus forked by a standalone developer into 2 variants:

• Classic variant preserves the original look and features
• Plus uses more modern methods with additional features, one of which being portability.

The "Plus" program is "freemium," with extra special (usually non essential) features for the Plus variant available for purchase.

Note: the official homepage's download page tends to be outdated, so linking to GitHub's releases instead.