My recommendation, and the format that I've used several times to good effect, is to use a wiki.
There are a number of free wiki sites available out there. I've been involved with wikis on pbwiki, wikispaces, and Obsidian Portal. I'll put some links in this post later on (although my links page will have many of these already.) I've only ever used pbwiki as a player in someone else's game (and that wiki is not public, so I can't link to it and have it be something you look at) but it seems pretty comparable to wikispaces.
Does it matter which one you use? I don't think so. Obsidian Portal is more overtly gaming focused, so it has a few "tabs" that are useful. I also like some of the layout options there. However, there's no good sidebar menu option for the wiki. On the balance, I think I actually prefer wikispaces, but honestly only by a slim margin. Both options (as well as pbwiki) are free, both can be set up as public of invitation only, and for both you can add all kinds of detail to your setting to your heart's content.
But waitaminute... I didn't say a setting wiki. I said a campaign wiki. The Aquerra wiki, a wiki by the guy who sold me on the concept of using wikispaces and wikis for campaign management in general (Osvaldo Ortega--nice guy), is a setting wiki. If you click on the sidebar menu item Campaigns, you'll see that there have been no fewer than 15 campaigns run in the Aquerra setting, and this wiki is for all of them... as well as other setting material that isn't necesssarily directly relevant to any of the campaigns. That's a pretty ambitious wiki. I certainly don't recommend that you start out that way. However, for long-running campaigns, or settings that have multiple campaigns in them (especially if there's overlap with the players) then you can grow a wiki to be this big. But I'm going to show you two examples from my own gaming that are much more modest... and which ended up being completely sufficient for my purposes.
The first one is for a finite campaign that finished well, called Demons In the Mist. This was my first campaign wiki, and it was a rather slap-dash campaign that was literally kinda thrown together as it was being run. I started with very little material for the setting, and frankly, I didn't necessarily add much to what I started with. Let's go through the material on the wiki real quick. Look at the side bar, and we'll follow along, with some discussion, from top to bottom.
- Home. This takes you to the main page of the wiki. Always have this be the first link on the menu, anyone on the wiki can easily get right back to the beginning. I also recommend, although this is a more recent refinement on my part, that Home page be the campaign brief, or at least a good chunk of it, if it isn't put somewhere else.
- Places. This link takes us to a small, simplified map of the area. This page also has a number of links to other pages that describe some of the small countries and territories throughout the setting. Note that this isn't big. I'm not mapping out the entire world here, like a gigantic Forgotten Realms map or anything. I have a modest map with about ten links on it to some more detail--and some of those were added after the campaign started. If you follow the links to the individual countries, you'll also find that most of them have little more than a few paragraphs of description. It's neither necessary, nor even useful (nor is it a good idea period, in my opinion) to give too much detail. Just enough to give the PCs a flavor for what the region is like.
- People. A list of PCs and NPCs, and a link to some info about each. For NPCs, I certainly don't have character sheets or other mechanical details. In fact, my players create and wrote most of those pages.
- Religion. I included this just because some weirdo mythological like religion is interesting to me as a fantasy setting fan. This could be religion; you could also have any other details about anything else about the setting that's important or relevant for the players to need to know--Organizations, Goverments, Religion, Customs, Local Music Traditions--whatever you think is important.
- House Rules. My game was predicated on being "weird stuff within D&D and not being standard stuff in D&D" so I had to have some specific inclusions and exclusions.. as well as a handful of minor houserules. This is all on a single page, and prints out to only about half a page. How many house rules is too many is a discussion for another post, though.
- Group Spoils. This was another page created and maintained by the players, just to list interesting stuff that they'd come across. It wasn't always up to date, but nice idea.
- Other links. Pretty much exactly what it sounds like.
For another successfully used campaign wiki I created a few years ago, check out my Freeport campaign wiki. In some ways, this is even more sparse than the earlier one (although that's also in part because the campaign was shorter and had fewer people involved.) Just as another example. The Jinkies! page was created by a player who wanted to sort and list all the weird clues about weird things that they found out in game. The premise of that game was sorta like a fantasy version of The Hangover--the PC's started off the game in prison with no memory of the last month or two, and had to gradually uncover what was going on around them. Because of that, clues were more important than normal, and keeping that sorted out was a helpful addition to the wiki.
In general, I think that's the secret to a successful campaign wiki. Be fairly light, quick and dirty on setting detail, give your players access to update the wiki, and encourage them to use it in the ways that best fit their needs. Tracking loot, clues, NPCs, or other campaign notes is more important than trade routes in an out of a city, or detailed lists or shops and taverns. Create only what you think will be useful in the immediate or near future, and don't spend time or energy going beyond that. If you do, there's a good chance you'll never finish with the stuff that you actually need to run the game. You've gotta show some discipline, and you've gotta show some focus. But if you do, you can create a tool that will keep you and your players engaged with the game on an ongoing basis.