Stride (previously HipChat) vs Mumble

You can access HipChat from pretty much every common platform. HipChat even allows chatting via SMS.

You can ping people to get their attention even if they are not online by @mentioning them. Depending on how the person has set their account up it can be by notifications via in-app sounds, visual alerts, and even email, SMS, or mobile app push notifications.

Including Asana, Github, Zendesk, WordPress, MongoDB, TeamCity, JIRA, Confluence, PowerShell and more that 40 others.

Free with unlimited users, $2/user/month for voice + video calls, screencasts, full history retention and management. (Even enterprise feature like SAML)

An intuitive, easy to overview interface listing both online and offline users.

Things like photos get automatically embedded in the chat thread once you upload them.

You can invite people to join conversations even if they don't have a HipChat account. HipChat will generate a link to share and the guest will only need to enter their name to join the conversation.

HipChat has an optional desktop client powered by Adobe AIR.

HipChat audio and video works on mobile platforms and web browsers.

Participate and keep tabs on ongoing discussions on particular topics or amongst certain groups of people.

If you are worried about third-parties getting access to your data you should consider self-hosting. With self-hosting you are in control over where your data is stored, who has access to it. You will also not be vulnerable to exploits of a third-party provider.

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A big company with experience will bring in more options like Google Apps One Time Login.

E.g. part, away, all, here etc.

Mumble is available as a free download and source code is available on GitHub.

The server software Murmur is licensed under a free and open-source BSD-style license.

Available on Windows, Linux, OS X, Android and iOS which allows for the user to choose what operating system works for them and still be able to use Mumble.

Since version 1.2.4, Mumble uses an audio codec named Opus that's specifically developed to provide low-latency voice communication over Internet. This means that there is no delay time, while audio still comes out clear.

Dedicated servers and low overhead allows Mumble to have the least delay between users speaking and others hearing their speech, which can be very important for fast paced games.

A push-to-talk key allows you to only transmit audio when you want it. Others won't hear you chewing or your mom entering the room unless you want to.

Mumble is completely free, even hosting your own server.

Mumble uses certificates for user identification. A certificate can be generated by the user himself or issued by a third-party organization with organization certificates having 2 classes. A self-generated certificate is not tied to anything, a Class 1 certificate is tied to an e-mail address and a Class 2 certificate requires proof of identity. Depending on the server a different level of certification might be required.

Mumble server side is easy to setup and works smooth even on Raspberry Pi, which is pretty short on resources.

An in-game overlay can be enabled to show users in the channel or users talking. Overlay can be disabled completely or on a per-game basis by going to Settings->Overlay. This allows for users to see their chat but not obstruct the game they are playing.

By default, only users in the same channel can hear each other. Channels can be nested to better organize yourself according to what you want to be talking about.

This encryption is mandatory and cannot be disabled. The so-called control channel, which transports your chat messages and other non-time critical information, is encrypted with TLS using 256 bit AES-SHA. The voice channel carrying speech and positional audio is encrypted with OCB-AES 128 bit.

Mumble includes great automatic volume adjustment functionality that can be used to prevent big volume changes while still allowing good communication.

Mumble supports positional audio for over 50 games, including versions of Call of Duty, Battlefield and Counter-Strike. This allows you to hear the audio from the direction of the player speaking, making gameplay more immersive and giving competitive players an edge. You can find a full list of supported games here.

Version 1.3 introduced skins, allowing you to customize your UI, either on your own (tutorial here) or by using one of the skins found here.