A couple of tips that may be handy:
1/ Fractal Terrains (by ProFantasy). It ain't shareware, but its not that expensive, and it does produce good results if you tweak the supplied setting files a bit (I can maybe give you a few pointers if you get it)
http://www.profantasy.com
There's a demo you can download and see if you like it. I've used it for creating worlds. moons, and clouds, using different settings for fractal body generation and colour.
2/ Flaming Pear's Lunarcell - it apparently works with PSP as well as Photoshop. Get the 20 day demo and have a play. It does LOVELY craters, which I use to bumpmap moons.
3/ Wilbur
http://www.ridgenet.net/~jslayton/software.html
Wilbur is a heightfield manipulation tool that allows you to input geryscale bumpmaps (8 bit) and converts the shading to height. This is useful for doing altiutude-based colouring for world maps, and also for seeing roughly what the bumpmap will look like when illuminated.
You can get some very interesting effects by combining a greyscale Fractal Terrains-generated map with a lunarcell cratered bumpmap. the level of variance gives a nice random feel to the relief of the world or moon your creating.
As for Gas giants, the traditional method seems to be to manipulate maps of those existing in our solar system. If anyone has any tips on this, I'd be interested in reading them, especially those who have generated fresh gas giant textures.
Hope this helps,
Cormoran
'...Gold planets, Platinum Planets, Soft rubber planets with lots of earthquakes....' The HitchHikers Guide to the Galaxy, Page 634784, Section 5a. Entry: Magrathea