With the release of Microsoft Lync 2010, we are introducing Lync Web App, a new Silverlight based web browser client that allows participants without Lync 2010 account or Lync 2010 client to participate in meetings using Windows or Mac operating system and present and view PowerPoint slides, share desktop/application (Windows platform only), poll the audience and collaborate on a whiteboard.
Lync Web App can be used by employees who have Lync 2010 account, but do not have a Lync 2010 client installed on their machines, like those working from home on a PC or a Mac. Lync Web App also enables external partners, such as salespeople, who are invited to Lync 2010 meetings but do not have Lync 2010 account to join meetings as guest.
The Lync Web App sign-in page below illustrates how Lync Web App can be used by guests who do not have a Lync 2010 account, as well as by users who have a Lync 2010 account, but do not have Lync 2010 client installed on their machine.
Read more at source: http://communicatorteam.com/archive/2010/10/19/1763.aspx