GIfTI
GIfTI is the Geometry format under the Neuroimaging Informatics Technology Initiative (NIfTI). Basically, it is the surface-file format complement to the NIfTI volume-file format .nii. Programs which support the Gifti format, intended to allow exchange of each others surface files, include: Freesurfer, Caret, BrainVISA, Brain Voyager, CRkit, VisTrails and AFNI.
In Freesurfer, the GUI utilities freeview and tksurfer can open and display .gii files directly. The tool 'mris_convert' can be used to convert between native Freesurfer surface formats (like lh.pial) and gifti (lh.pial.gii) for exchange with other packages. See the output of 'mris_convert -help' for examples. ie. mris_convert lh.pial lh.pial.gii