The Personal Software Process (PSP) is low overhead, high maturity software development process targeting tasks done by a single developer.

Six Sigma is a measurement - driven approach to continuous improvement that focuses on reduction of variation, consistency and predictably high quality.

The Team Software Process (TSP) adds high maturity project planning and management processes and when used with PSP, provides a high maturity development process for Software project teams.

Six Sigma provides statistical analysis and decision-making tools that are not built into PSP/TSP.
PSP's training program and metrics framework provide the infrastructure required to get the quality measurements as envisaged by Six Sigma approach.

Six Sigma and PSP/TSP can certainly be used independent of each other. There is a natural synergy between them, with each acting as an "enabling" Technology for the other.

Both PSP/TSP are available under License from the Software Engineering Institute (SEI)  More