How can I open an Adobe photoshop/Illustrator files? [FOSS Software]

Besides the commercially available (costly?) Adobe Photoshop and Illustrator, there are ofthen freeware graphics application program, These can often open/edit files like *.psd, *.ai.

Gimp is an alternative to various Photoshop Processing Requirement
Skencil is an alternative to various Illustrator Processing Requirement
Inkscape is an open source vector program.