"A picture tells a thousand words." Sure, it's a cliche but, like a lot of cliches, it's surprisingly accurate. I don't start a new campaign unless and until I have some art that gets me in the right mood and right headspace to run it.

Published adventures are number two on my list. They provide a structure from which I hang my own ideas and the ideas that are developed in play.

Finally, I normally run FR campaigns so I love digging into the back catalogue for anything written by Ed Greenwood or Eric L Boyd, or digging up relevant material at the Candlekeep forums written by the two Es and by George Krashos or Brian Cortijo (Cormyr expert).

Mostly fantasy books, but also history, music, poetry (I once based a campaign on ideas I got from listening to Bruce Dickinson's Chemical Wedding, and he was inspired by Blake), films, even dreams! Anything I can grab a useful NPC, plot idea, item or threat from. Published adventures are great too.
I wrote a WW2 CoC con scenario about an American B-17 crew after reading the (non-fiction) book "Mighty Eighth", and another one based on the album "The Big Red Spark" by Tinyfish. I'm just a sponge for ideas, really!

Mostly written material, of all kinds.  Occasionally, a spoken fragment from a poem or a similar fragment from a song.  

It is very rare for me to be able to directly trace the inspiration for even part of an adventure, much less the whole thing.  I think I've done an "inspired" adventure twice in the last 20+ years.  What usually happens is a mixture of many different sources.  I'm sure that some of it is prompted by particular sources, but it can become difficult to tease out the influences.  For example, my style of adventure is certainly influenced by the "Zimiamvian" cycle by E. R. R. Eddison ("The Worm Ouroboros", et. al.)  Yet, I doubt I've ever lifted even a niche character from it directly, much less a story line or setting.

I could mine adventure ideas directly from Samurai Jack and not run out for years.

Quote from: Joey2k;1105468Random tables. I love rolling up a bunch of stuff on several different tables and then trying to come up with some cool way for it all to fit together.

This.  I'll draw inspiration from anything I see, like most.  But what underlies those bits and pieces seen or read elsewhere are usually good table results causing me to go outside of my natural thought processes.