Well you can get free 30 day trials of Adobe Photoshop, JASC Paint Shop Pro, and ACDSee.
For free you might want to look at irfanview here: http://www.irfanview.com/
Though it's mostly for display and conversion rather then a tool for creating art from scratch, but it should work great for resizing and converting images to .pcx for use in GalCiv.
Maybe some others can recommend free paint programs / tools / filters.
Dunno about 3D programs... on the commercial front there are many, but I have used Bryce 3D to make beautiful skies and landscapes and find it "easier" to use then many others. Rhino 3D is a good 3D program also, but has a bit of a learning curve. (there are tons of others like lightwave, poser, raydream, maya, etc. etc.)
There is a Japanese shareware proggie called Doga3D which is absolutely spectacular, but a bit "simplistic" for a serious 3d person (which I am not, so I like it heh

). Version 1 is free but the better versions 2 & 3 require a registration fee. Here's the link:
http://www.doga.co.jp/english/
Hopefully someone else can post some good freeware programs here for you!