I'm mostly-kind-of-almost finished with VALIANT, a YA retelling of a The Brave Little Tailor. For the past year, I've been living in– and creating!– a fantasy world that I hope readers will want to enter. I want it to be so real, so vivid, that someone will feel homesick once the story's finished.
I'm still learning this worldbuilding thing, but I did gather a few tools along the way. There are tons of websites with worksheets that help you expand your world's culture(s). (Google 'worldbuilding.') However, I found I needed the most help with naming and visualizing my world. So I'd to share what I used for:
Character names
See Kristen's excellent post! (Or ask Craig. I wrote a few weeks before giving my MC a name– and it was finally Craig who suggested Saville.)
Place Names
Wikipedia is your friend! It wasn't until I'd gotten through most of the story that I realized the river in VALIANT felt like the Krivija, a river I loved in Bosnia. But I also based the river on the Danube. So I looked up the Danube in Wikipedia. The article provided a ton of place names that I could adjust and use. I can't tell you how many times I've sifted through Wikipedia for place– or character– names.
The process reminds me of looking through a thesaurus. You may not know what you're looking for, but you'll know it when you see it.
Fantasy Language
VALIANT needed a few words in the giants' language. I felt completely overwhelmed until I found Definitions. I'd type in a word like 'little' or 'traitor,' click 'Translations' on the right, and then see the word translated into about twenty different languages. (I LOVE this website!) Often, I'd find a word that sounded almost right. Then I'd play with sounds and syllables it was just what it should be.
A word of caution: be sure to keep the sound or feel similar. You probably wouldn't want to claim that Hindi and German-sounding words come from the same language.
Rambling ahead:
Music
It isn't easy finding a playlist for a fantasy. But never fear! Use Pandora. For those of you who aren't familiar with the website, you can create a station by suggesting songs that you like and then rating songs that the website plays. It takes a little time, but it's worth it. And it has access to movie soundtracks, which was perfect. After a little tweaking I created a station of soundtracks and composers that matched the feel of VALIANT. (It's a combo of Lord of the Rings, Last of the Mohicans, Gladiator, Braveheart, etc...)
May I add that while I don't daydream about Oprah mentioning my book, I DO fantasize about Peter Jackson making it into a film? Two reasons: 1) He'd film it in New Zealand, and I'd have to visit at least once to squeal over the sets, and 2) I want to hear what Howard Shore would do with the giants' songs. I don't know how they sound– just how they make the listeners feel. But I bet Shore could figure that out....
Annnnd... back to our regularly scheduled programing:

After a while, I needed faces for my main characters. I knew roughly what they looked like, but I wanted pictures. And I knew Lord Verras did NOT resemble any member of One Direction. I needed more than a Google Image search. So I went to London's National Gallery website and searched the paintings by century. It was like searching a crowd for a friend's face.
I thought about sharing the portraits here, but I don't think they'd work. For me, the search was as much about the emotion in the portrait as the face itself. Still, I discovered such interesting things about my people! For instance, I knew one character was chubby but I didn't know he was swarthy until I saw his portrait.
I know there are a million other resources out there. I'd love to hear from you: What have you used as you write fantasy?