Slack is just IRC stuffed in a new bottle is a communicator for teams, as an alternative to hangouts, jira and yes IRC. They do have integrations with Google drive, and many other web services which makes them stand out.. Now they are giving out $100 in free credits. ( and yes there is also a forever free plan for unlimited people).
https://slack.com/r/025cnc7d-025cnc99
PS: This is an aff link, and the promo is only valid via referals.
Update : if you answer a 6 qn survey right after the account creation, you get another $100 credits making it $200 in total
The credits page states this
Your credits will expire after 60 days if not used.
however do not panic. the faq states
**My team's credits are going to expire before we can use all of them. That sucks!** As long as you sign up for a subscription plan before the expiration date, those credits are yours to use until they're all gone.
And if you do not use an account for more than 10 days, they will stop charging you, so the credit stays forever in case it the account is kept idle.
Here's how it works: Slack does a daily check for activity on every one of your team's enabled users who have had their accounts for at least three weeks. If we find a user who hasn't used Slack at all in 10 days, we will consider their account to be inactive. Likewise, we'll do a check on inactive accounts to see if they've become active again and, if so, mark them as active. When you upgrade to a paid tier, you’ll only be charged for accounts that are currently active. Any changes to the number of active users during your billing cycle will be result in a pro-rated credit or charge, just as they to if the accounts were manually added or disabled by an administrator (described in the previous two questions). The only difference is: you don’t have to lift a finger.