Is there any online resource for sketching out maps to be used in 40k battles or campaigns? I've been told so, but so far I've turned up nothing.

Ive got this...sortta okay program called Hex, world creator, which uses hexagons to make a map. It works alright, and im trying to usse if for a DnD campaign, but every five minuted it sorta cancels out, so i have to save alot. But that might not happen with you.
Just type in hexic, world creator in google.

And tell me if you find a better program!

MS Paint? :P

I drew up this little map during lunch today. It's more fantasy oriented but some new object could be thrown in as Sci-Fi markers

http://i262.photobucket.com/albums/ii91/Razal1983/testmap.jpg

used CC3. Has a feel of a CAD or similar program, but for making campaign or RPG maps.

Once I get better at using it I was thinking of doing comissions. This was my first attempt.

MS Paint? :P

You know, I completely agree with ZT. Maybe I'm just old-fashioned, but Paint is, was, and always will be King. I usually just do a google image search for aerial photos (or photos of space if it's a pretty large-scale campaign) and then just add in some borders with Paint.

Here's an example of a map I made for a Necromunda campaign - if you're curious, the triangles are passages down to Underhive territories, which is another map.

http://i138.photobucket.com/albums/q255/dirgebringer/hivecity.jpg