Found a page listing the variety of open source and free software that one can use, e.g - Sun's Star Office is very reliable if you are looking for a free alternative to something like Microsoft's Office (Word processing, spreadsheets etc.); or one could use Gimp for graphics tasks instead of something like Photoshop and if the needs are for 3D, there is Blender which personal friends of mine highly recommend.

See list here, courtesy of John Hopkins University.