All-in
one: Fire eliminates the need to have multiple chat programs open at once
to use multiple chat clients.

The window entitled "Buddies"
neatly stacks all your buddies together, no matter what service they are
using.

The "Edit
Buddies" window allows you to customize everything about each individual
buddy.

Fire's chat
windows feature everything from a built-in smiley menu, which shows
all the smileys each service will support, to built-in translations.
The
"Edit Away Messages"
window allows you to customize your away, busy, and idle messages in many
ways.
The"Fire
Console" keeps track of what happens while the program is open.

And "Stored
Instant Message Conversations" keeps track of what your friends
said so you have some evidence against them when they deny it a week later!
Created: Thursday, July 24, 2003
Updated: Tuesday, September 30, 2003
I've been using Fire for over a year now, and I think it's the best multi-client app out there. It overcomes many of the shortfalls of the official clients (for example many don't have message logging), and adds features such as auto-translation (now you can chat with a foreign girl/guy by typing English and them typing Dutch/French/German/Spanish/Italian/Portuguese), name changing (you can make your buddy John appear as "John" instead of "xxSexyGuy22yy"), transparency, auto-reconnect, console (which of your friends signed on while you were away?), and many more. It currently lacks a few things that the official clients have, such as MSN's group chats, AIM's voice chat, etc. However, this program will put you in control on any or all of the following: AIM, MSN, ICQ, Yahoo!, IRC, and Jabber. And, as if that wasn't enough, it's free!
The developers are extremely responsive. I remember once I was experiencing problems with MSN and I told the developers (who were hanging out in the official Fire IRC channel) about it. Immediately several attempts were made to solve the problem, and after about two days everything was running fine again.
Some of the tools that Fire has are extremely helpful. As I mentioned before, the console window, which is probably overlooked by many, is great for seeing who signed on while you were away. It's actual purpose, I believe, is simply to monitor your connection status and give hints as to what the problem is if one of your accounts don't connect. It is much more useful than simply that for me.
Offended by vulgar language? Fire has a profanity filter! Not even AOL with their pathetic parental controls advertisements has that!
Fire can also fully utilize Apple's speech software. You can have any Apple voice tell you when a buddy logs on or off, and even have all your messages read to you.
Fire also has a whole set of individual controls for each of your buddies. You can prioritize which windows appear atop others, and how you are notified by each buddy's arrival and departure, if you wish.
The "previous conversations" feature is extremely useful. Let's say I'm chatting with some girl. Let's say I talked to her about some issue a few weeks ago, but can't remember exactly how she felt or what she said. Instead of having to ask her, which might give her the impression that she's not that important to me (because after all, I forgot what she said), I can see all my previous conversations with her with one simple click on the toolbar. It can never hurt to score a few extra points! Heheh…
Simleys are customized for each service, so your drop down bar displays all the simleys your buddy is capable of seeing if they are using the official client.
The accounts window lets you easily organize all your accounts from one window. At the time Fire only supports one connection per service, but it looks like this will change in the future.
The buddies window also has many customization features. As I mentioned before, you can "alias" your friends so your list looks neat and you quickly know who's on. This is extremely useful for MSN, because MSN's official client will just display a buddy's display name, which often doesn't tell you who someone on your list is. Then you have to hold your mouse over the name which yields their email address, which often doesn't tell me anything either! You're probably not the only one you know using more than one network either. That's where Fire's clustering features becomes very useful. It will compact all your buddy's online appearances into one. So if you have a friend who's on AIM, MSN, and ICQ, Fire will compact him or her to only appear by your preferred service if you wish.
AIM is probably best supported by Fire. Just about everything important but voice chat and the sending of buddy icons (coming in the next release) is supported. Yahoo! is next in my evaluation of best-supported. MSN and IRC are probably least supported, but still well enough to not cause many problems. MSN lacks group chat and of course more recent things such as video chat. But that's not even supported by the official client for the Mac, or Windows 98 for that matter!
There are some other features I haven't even messed with, such as the secure encryption feature. And I have most likely forgotten a few. But Fire is a great program, and it is easy to tell the developers have put, and continue to put a great deal of time into it. It is undoubtedly the best multi-client program for OS X.
See Also: IM Wars (Review of IM Neworks)