"The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts." Bertrand Russell
As well as the software listed on the page CaptainHero's link points you to, there's a program called VistaPro which is similar to Terragen - my copy came off a magazine cover CD but I expect you can download it too. Also PhotoImpact is another alternative to Photoshop for image manipulation. HTH.