So I get asked this question a lot, and I’m tired of explaining it. Leeching various (legal) things from IRC is not difficult. So, let’s get started.
Introduction
IRC, or Internet Relay Chat has been around for ages, and many still use it today. If you don’t use it, you’re missing out. This is not a Yahoo chat, nor an AIM chat. Most channels are controlled by bots. (Programs placed on servers) These bots do various things such as automatically banning spammers, holding the operator position in the channel, or maybe even trivia. There are also other kinds of bots, one is called an XDCC bot. They are in certain channels where you may download various things.
What’s needed
First, you’re going to need an IRC client. This could be mIRC, which is probably the most popular, XChat, Irssi, or anything else that connects to the IRC protocol and has DCC (Direct Client to Client) functions. I am going to use XChat for the example, because it pretty much works well with transfers right out of the box.
Start up XChat, click do not display this message again when the first pop up box shows up. Exit out of that.
Basic commands
/server irc.xxxx.tld – Sets the irc server you wish to connect to. (Example /server irc.technologyinsanity.com)
/join#channel – Lets you join a channel while connected to a server. (Example /join #technologyinsanity)
/msg USER <message> – private messages a user on the network you’re connected to. (Example /msg s0uThp4rk what’s up?)
Network Services
Some may require you to register with nickserv or get a vhost (a virtual host, which would be s0uThp4rk@the.cow.com instead of s0uThp4rk@myip.resolvedhost.tld)
In such cases, here are some basic commands for that.
/msg nickserv register <password> <email>
Then, generally you will join #vhost and send the message !vhost your.vhost.tld
Requesting Packs
In order to request a pack from an XDCC bot, you must be in the channel that has such bots, and meet the requirements for the bot if there are any. You may use an XDCC search engine such as packetnews.com or xdccing.com. I would not recommend you use these websites without adblock plus enabled. You can then click on the pack or one of the other links and it will try to automatically open XChat and join the channel. An alternative would to idle in a channel and request a pack you want when you see it. You could also install a script to “catch” xdcc packets and request them from a window within the script.
The general command for requesting a pack is:
/msg <botname> xdcc send #<packnumber>
Then you have to accept the send, or wait in queue until an open slot is available. You can also set xchat to automatically accept sends in the preferences.
Conclusion
That should be about it for this introductory. Instead of leeching things from xdcc channels, maybe you would rather chat? Check out my channel at irc.technologyinsanity.com #technologyinsanity 
Packs on XDCC. These are not legal, however..
Irssi is a very interesting IRC client. It runs completely through the command prompt. It runs on Windows, Linux, and Mac OSX. On windows, you can install it through the executable. It will run through DOS/Putty command lines. On Linux, you can either compile it or install from a binary, through apt-get or something similar. It runs through Bash, obviously.