Lync is an instant messaging client used with Microsoft Lync Server or Lync Online available with Microsoft Office 365 and Live@Edu.
Ranked in these QuestionsQuestion Ranking
Pro Video conferencing
Lync allows real-time communication for up to 250 people at once. 5 of them will be able to use Full HD video.
Pro Collaboration tools
Pro Deep Microsoft ecosystem integration
Lync integrates well into the Microsoft ecosystem (Exchange/SharePoint/Office).