I use Anim8or for most of my models. For post-render work, Gimp is a free alternative to Photoshop that can do almost everything I want (except sharpen fuzzy images).
Celestia is best for planets that need particular surface characteristics, and
Space Engine for more generic ones. Space Engine also makes nice random planetary surfaces which can come in useful. I have used Torben Mogensen's Planet to create surface textures for planets, but for many planets I use a demo version of
Fractal Terrains. Sometimes I use a demo version of
Terragen as well - it takes a long time to render, but the results are worth it.
Quite often I use chopped-up bits of the Earth as a basis for Earth-like planets, coupled with an ocean texture generated by Fractal Terrains or Planet.
Gas giants can be quickly put together by chopping up large resolution images of Jupiter, which is almost infinitely variable in texture over time.
Incendia makes some nice 3D fractals, which can be used for Dyson Spheres and other megastructures, but it has a long render time too. The other free modeller I use sometimes is
Sculptris, but it takes a lot of patience to get what you want.
I do have Blender, but rarely use it - still too much to learn. But it can probably do everything I want all in one package. Maybe eventually...