Texturemaker has some really nice heightmap tools with it. If I am not mistaken the demo will let you save textures up to 1024x1024.
You can try out the enterprise functions too for up to 30 days.

They have the "next day" engine so you can see a quick preview of your map. After you get everything looking OK, you can use the advanced texture mixer to make the diffuse map.

under the Generators.perlin amp, there is a few heightmap functions for mountains, roads, building sites, and rivers/lakes.

I bought it back in 2002, i have used it almost every day since. EVERYTHING is made for seamless textures.
play with it HERE:
http://www.texturemaker.com/download.php

you can script it too! so all you server owners out there rejoyce! you can make scripts to randomly make bases, and blend them into the terrain without cliffs/vallys on the edges! It also runs "out of the box" with WINE, so doing a linux server is really easy. (not sure if AOS runs under wine yet or not, i am using virtuabox, and Win7 64bit VM.)

There are some heightmap tools for GIMP too.Not sure where to get them for the Windows GIMP. I get them from the Ubuntu Apt-repository.

bought, you mean you have to pay for it? No deal.

Bump x Infinity

JoJoeStinky's maps are Ill as hell, and everyone interested in making maps should experiment with his methods.

Thanks MegaDeuce!

You inspired me to make a tutorial here:
http://ace-spades.com/forums/topic.php?id=5686

Just added a video the above thread :)