There are many photo editing softwares available in the market but photoshop is the best software.
It is the software loved by most pc as well as mac users.
Version 5.5 comes with a history palet that gives user unlimited undos. Lot of filters available and the right combination produce unbeliveble results.It supports large no. of file format.
If you have good sence of color, tonal values then let your imagination do the miracle and if you have a system like power mac g4 then sky wont be the limit.
Help provided is very helpful but it says the basic operation, you can creat amazing results only with your imagination.