Tessella™ is a plug-in for Adobe Illustrator 8.0 or later. It will work on both Windows 98 onwards and Power Mac 8.0 or up. It is an easy program to learn and produces tessellations in all 17 wallpaper groups. It can also produce symmetries in all but 1 of the 7 frieze groups.
The process is usually completed in 5 stages using the interface shown on the right. First a symmetry type is chosen and a tile produced using either a given shape or by using a path, or paths drawn by you. Now this can be edited in stage 2 (called Escher edit) by adding, removing or moving points and bezier curve handles in normal Illustrator fashion. However, to conform with the symmetry group chosen in stage 1 some points will remain fixed. These are clearly marked with green squares. The tile updates as you edit it.
Having produced a tile you are happy with, in the third stage you use the 'create lattice' command to duplicate the tiles and move, flip, mirror, them into a repeatable lattice group according to the symmetry type chosen. In the final stage, this group is repeated to fill the plane and you are finished.
Note that it is permissible to add artwork to the tile in stage 2 and it will be repeated as well provided the whole lot is selected before the lattice is produced - thus adding eyes and ears is easy! The program comes with a pdf manual which also covers a smattering of the theory involved.
How do we rate this program? Good. You can get results quickly. Easy to learn. Representational tessellations (animals and such like) can be produced with practice. Cons - needs expensive Adobe Illustrator software to make it work.
Shareware (cost $40) from Stephen Vincent here