I've been playing with Slack, an incredibly popular communications tool described as no less than an "email killer."

The company behind it was cofounded by Stewart Butterfield (who co-founded Flickr, another service I love), and hit a $1 billion valuation in October, even though it's barely been around a year. Slack has over 120,000 users across more than 2,000 organizations, and those numbers grow five to ten percent each week.

But what are those users doing? What exactly is Slack?

I practically run my life out of my Gmail inbox, so an "email killer" sounded like something that could turn my world upside down. As it turns out, Slack has a much more focused purpose than email. What it could kill, as far as email goes, is the inevitable nightmare that is email within an organization.

Who hasn't pulled their hair out over a Reply-All company-wide thread that refuses to die? Or missed a meeting because you weren't copied on the announcement? When it comes to email within a team, it often causes as many problems as it solves.

In buzzword terms, Slack is an instant, accessible, searchable platform for group communications. It's all the rage among startups, for sure, but even old-fashioned corporations are using is as well.

You could call Slack "groupware," but it's not the '90s anymore. You could call Slack a "group chat" service, but that would be overly simplistic -- after all, a company might already have instant messaging via Skype, and that can still be a mess. It could smell like a message board, but that conjures visions of clunky corporate intranets. I want to compare it to a private IRC server, but who the heck knows what IRC is anymore? Besides, Slack is actually easy to use, user friendly, and works beautifully across computers and mobile devices.

Slack works for groups of all sizes. A company or a department, a club or a social group, a distributed development team or students in the same classroom. Accessible via the web and native mobile apps, you can easily stay in touch. And unlike email, which is a 40 year old technology we're still bending and twisting ourselves to work around, Slack was built to help people work the way they work today.

Think of the worst-case Reply-All email scenario. Everybody might want to know about something, but not everyone needs to know about it, some people might not want to hear about it, and someone who was supposed to get the message will inevitably be left out. You have email groups for each department, meanwhile, but departments have to work with each other, and not every member of each department is involved in the same projects. Email messages fly in every direction, pile up in inboxes, and managing you inbox is suddenly a job on top of the job you're supposed to be doing.

With a Slack group, you get channels. You can have channels for departments, for specific projects, or for any sub group within your organization.

For example, there's a #general channel that everyone is in by default, so you can tell everyone about the coffee cake in the lunch room. There's even a #random channel for those funny cat photos. Meanwhile, everyone working on the new marketing plan can be chatting away in #marketing, without bothering anyone else.

But when the guys in #development want to know how things are going with the next ad campaign, they can click over to the #marketing channel to catch up (and vice versa). Of course, it's also possible to create private channels (be it #surpriseparty or #worlddomination).

And everything is searchable. Who was that guy we met with yesterday? Which restaurant did the boss like? Ah, there it is.

And in place of person-to-person emails, you can send direct messages to other team members. But because these messages look and feel like instant messages (like SMS or IM), interactions are lighter and faster. You don't get bogged down in salutations and clever signoffs and making your point with three well-crafted paragraphs. You just say what you have to say and get things done.

Fortunately, Slack isn't just for typing. You can post photos, PDF files, self-contained posts and code snippets, for starters. And because this is a modern tool, there are many direct integrations ready to connect to the other modern tools your team might use: Drobox, Google Drive, GitHub, and more. Slack doesn't try to do everything or replace anything (except, I guess, email), so it plays nicely with seemingly anything else you already use to do your work.

Again, though, Slack isn't just for work. I got interested in it because a friend of mine who plays Ingress said it was what his local team was using to both strategize and talk story. People set up Slack groups for one-off projects like planning a conference or a wedding. And yes, you can be a member of more than one Slack team.

Finally, the kicker? This immensely popular tool that is playing a pivotal central role in many companies is free. Declares the homepage: "Slack is free to use for as long as you want and with an unlimited number of people." In other words, the basic functionality of Slack is available for you to try or even use at no cost.

Of course, Slack wants to make money, and it does, even at a pricey $7 per user per month. And the only difference between a free and a paid Slack account is how far back you can search (you can go back 10,000 messages for free), more storage (5GB is free) and how many integrations you can hook up (five are free). For some companies, Slack becomes so critical, it's worth paying for. But otherwise? You can jump right in.

As you can imagine, though, trying out Slack is no fun by yourself. So I created a Slack group at hawaii.slack.com and have invited as many cool Hawaii geeks, techies, and creatives that I can think of. If you'd like to join our motley crew, whether to just kick the tires or to hang out and talk story, just let me know your e-mail address and I'll send you an invite!