I have successfully used several methods to contain GM information in growing worlds. I have used wikis - including desktop and tiddly varieties, Scrivener, mindmaps, notebooks, card files, and more. The current champion is Evernote.
I have used Evernote to organize notes and information for my Dresden Files campaign - I turned to this tool when the MindMeister mind map I was previously using was overwhelmed by the number of nodes I was creating. Evernote has a large ecosystem of apps with integration, including Mural.ly if you want the visual layout and organization of a mind map or mind-map-like application.
I am presently storing notes and resources for a TimeWatch game. The killer app for this - and I have tried the new, free, multiplatform OneNote, just BTW - is the Web Clipper. See an interesting article? Clip it. See a picture of something you want to use as a setting? Clip it. A wikipedia entry on an animal you want to use as a monster? Clip it. The web clipper turns "that looks cool...I should remember that for my game..." into "that was cool, glad I grabbed it for my game!"
Evernote notes are formattable with bullet, numbered, and checkbox lists - you can write a list of information PCs should get from a scene and make them checkboxes and record whether they got the information or not...
They can contain links, images, and even audio. It will be a big file, but you could record your whole session in a note...
You can access Evernote from any modern computer (web interface) or mobile device (apps for phone and tablets).
You can organize your materials into different notebooks, use tags to mark campaigns, systems, settings, NPCs, monsters, etc., and use Evernote's amazing search to find everything fast.
Oh, and Evernote will make text in images searchable! Snap a picture of a book cover, a sign, or even a (moderately neatly) handwritten document, and after a sync period, you can search for the text inside. Fantastic.
Evernote is free for 60MB of uploaded data per month and you can buy extra capacity without committing if you just have a picture-heavy start or something. So far, I have never had to pay.
It's also great because it's meant to keep stuff forever. So stuff I squirrelled away for that Dresden campaign that was never used is now available - easily - if I want to grab it for TimeWatch!
Want to share with co-GMs? Evernote supports shared notebooks. Want to offload some responsibilities (like session logs) to your players without opening the kimono? Give them the email address for your Evernote and let them mail notes to you...
It makes keeping track of your world(s) quick and simple.