IRC began to catch on in the late 80s and early 90s, making it the oldest form of instant messaging still popular today. The apps in this section (and the next) are strictly Internet Relay Chat (IRC) clients. They are not instant messengers that simply handle IRC as well. I used the official Ubuntu IRC channel (#ubuntu at irc.freenode.net) to test whether or not these clients functioned.
The listings on this page are feature-heavy IRC clients. Most have a GUI option for essentially any action, which can make them more approachable to beginners. These apps also tend to leave a larger footprint on both disk space and system resources than those on the next page. Their Windows-equivalent would be mIRC.
As you might have guessed, Konversation is the default IRC client for KDE and this client is not lacking in options. Tabs are located at the bottom of the interface, so you can have multiple channels open at once. For preferences, you can change the color of pretty much every component on the screen. Even though the default is black text on a white background, you can set up the Matrix-inspired neon green text on a black background with minimal effort. You can also add and remove navigation bar (called toolbar) items a la Firefox. Bookmarks also work identically to those in a Web browser.
Konversation's 'pausing' system is the standard scroll-up. If the scrollbar is positioned at the bottom, the window will continue to scroll down as new text is added. Simply raising the scrollbar above the bottom will pause the chat, allowing you to read text at your leisure without fighting the scrollbar. Returning the scrollbar to the bottom will re-enable auto-scroll. This app also lets you manage multiple identities, each with multiple nicknames. There are even options for configuring custom keyboard shortcuts. Konversation manages to incorporate a sickening number of options, yet somehow still manages not to overwhelm the user. I'm giving Konversation the top spot for its straightforward, yet feature-rich interface. You'd be hard-pressed to find a better option on any platform.
First off, there are two versions of Quassel in most repos: a client-only version and a client/core combo. This covers the client/core combo, since the client-only version requires the user to manually set up the Quassel server, in which some command line work is involved.
This is a fully-featured IRC client with tons of options. You can adjust the color scheme, manage aliases, set keyboard shortcuts, and do minor modifications to the interface. The basic three-pane layout is used, making Quassel easy to pick up for anyone who has ever used a graphical IRC client before. Even new users should have no problem connecting to servers and finding channels using the GUI tools for server presets and channel lists. Since version 9.04, Quassel is the default IRC client for Kubuntu.
KVirc has many option settings, which is great (unless, of course, you're new to IRC and/or KDE).
There is also the issue of this app's look and feel. First of all, it has a splash screen and many instances of pictures set as window backgrounds. Thankfully, there are no obnoxious system sounds. There is a pervasive default background, an irritating beige slab with 'KVirc' written graffiti-style in white, that ends up becoming very distracting. Fortunately, this app has Configure Themes and Manage Themes options under Settings in the menu bar.
On the other hand, a quickly-accessible bookmarking system is lacking. KVirc also crashed multiple times on my test system's 64-bit Ubuntu installation. By running KVirc in the VM installs of 32-bit Ubuntu and Kubuntu I've managed to narrow down the buggy behavior to both 64-bit and GNOME issues. It didn't crash in the 32-bit edition of Ubuntu, but ran much faster in Kubuntu. KVirc has a ton of options, but doesn't organize them in a logical and easy-to-access fashion. In fact, everything with this app seemed clunky, and I often found myself trying to figure out how the developers intended me to navigate menus and apply changes instead of just doing it. Simply put, KVirc fails where Konversation succeeds.