Many applications require GHz femtosecond pulses with the center wavelength tunable over a broad range. Recently we have demonstrated a 3-GHz, femtosecond Yb-fiber laser system with >12-W average power using the pre-chirp management method. Nonlinear wavelength conversion based on this powerful laser source has enabled us to derive 3- GHz femtosecond sources at other useful wavelengths: using stimulated Raman scattering inside photonic crystal fibers, we have implemented a femtosecond Raman soliton source tunable between 1.06-1.35 μm; using fiber-optic Cherenkov radiation, we have demonstrated a 14-fs source centered at 850 nm with >300-mW average power.