Many projects offer real-time chat rooms using Internet Relay Chat (IRC), forums where users and developers can ask each other questions and get instant responses. While you can run an IRC server from your own web site, it is generally not worth the hassle. Instead, do what everyone else does: run your IRC channels at Freenode (http://freenode.net/). Freenode gives you the control you need to administer your project's IRC channels, while sparing you the not-insignificant trouble of maintaining an IRC server yourself.
The first thing to do is choose a channel name. The most obvious choice is the name of your project—if that's available at Freenode, then use it. If not, try to choose something as close to your project's name, and as easy to remember, as possible. Advertise the channel's availabity from your project's web site, so a visitor with a quick question will see it right away. For example, this appears in a prominently placed box at the top of Subversion's home page:
If you're using Subversion, we recommend that you join the email@example.com mailing list, and read the Subversion Book and FAQ. You can also ask questions on IRC at irc.freenode.net channel #svn.
Some projects have multiple channels, one per subtopic. For example, one channel for installation problems, another for usage questions, another for development chat, etc. (“Handling Growth” in Capítulo 6, Communications discusses and how to divide into multiple channels). When your project is young, there should only be one channel, with everyone talking together. Later, as the user-to-developer ratio increases, separate channels may become necessary.
How will people know all the available channels, let alone which channel to talk in? And when they talk, how will they know what the local conventions are?
The answer is to tell them by setting the channel topic. The channel topic is a brief message each user sees when they first enter the channel. It gives quick guidance to newcomers, and pointers to further information. For example:
You are now talking on #svn Topic for #svn is Forum for Subversion user questions, see also http://subversion.tigris.org/. || Development discussion happens in #svn-dev. || Please don't paste long transcripts here, instead use a pastebin site like http://pastebin.ca/. || NEWS: Subversion 1.1.0 is released, see http://svn110.notlong.com/ for details.
That's terse, but it tells newcomers what they need to know. It says exactly what the channel is for, gives the project home page (in case someone wanders into the channel without having first been to the project web site), mentions a related channel, and gives some guidance about pasting.
Many technically-oriented IRC channels have a non-human member, a so-called bot, that is capable of storing and regurgitating information in response to specific commands. Typically, the bot is addressed just like any other member of the channel, that is, the commands are delivered by "speaking to" the bot. For example:
<kfogel> ayita: learn diff-cmd = http://subversion.tigris.org/faq.html#diff-cmd <ayita> Thanks!
That told the bot (who is logged into the channel as ayita) to remember a certain URL as the answer to the query "diff-cmd". Now we can address ayita, asking the bot to tell another user about diff-cmd:
<kfogel> ayita: tell jrandom about diff-cmd <ayita> jrandom: http://subversion.tigris.org/faq.html#diff-cmd
The same thing can be accomplished via a convenient shorthand:
<kfogel> !a jrandom diff-cmd <ayita> jrandom: http://subversion.tigris.org/faq.html#diff-cmd
The exact command set and behaviors differ from bot to bot. The above example is with ayita (http://hix.nu/svn-public/alexis/trunk/), of which there is usually an instance running in #svn at freenode. Other bots include Dancer (http://dancer.sourceforge.net/) and Supybot (http://supybot.com/). Note that no special server privileges are required to run a bot. A bot is a client program; anyone can set one up and direct it to listen to a particular server/channel.
If your channel tends to get the same questions over and over, I highly recommend setting up a bot. Only a small percentage of channel users will acquire the expertise needed to manipulate the bot, but those users will answer a disproportionately high percentage of questions, because the bot enables them to respond so much more efficiently.
Although it is possible to archive everything that happens in an IRC channel, it's not necessarily expected. IRC conversations may be nominally public, but many people think of them as informal, semi-private conversations. Users may be careless with grammar, and often express opinions (for example, about other software or other programmers) that they wouldn't want preserved forever in an online archive.
Of course, there will sometimes be excerpts that should be preserved, and that's fine. Most IRC clients can log a conversation to a file at the user's request, or failing that, one can always just cut and paste the conversation from IRC into a more permanent forum (most often the bug tracker). But indiscriminate logging may make some users uneasy. If you do archive everything, make sure you state so clearly in the channel topic, and give a URL to the archive.
 There is no requirement or expectation that you donate to Freenode, but if you or your project can afford it, please consider a contribution. They are a tax-exempt charity in the U.S., and they perform a valuable service.