Lync is Microsoft's unified communication platform. It ships with a pretty awesome SDK that makes adding presence information to your WPF and Silverlght apps pretty easy. It's also possible to add presence information to your ASP.Net apps, but it's a little trickier. The first thing you'll need for WPF or Silverlight is the SDK. The installed assemblies should be located in Program Files (or the x86 variant if you're running 64bit) under Microsoft Lync\SDK\Assemblies.
You'll need Microsoft.Lync.Controls.Framework.dll, Microsoft.Lync.Controls.dll, Microsoft.Lync.Model.dll, and Microsoft.Lync.Utilities.dll.
Reference those assemblies and you're ready to drop in a control:
The PresenceIndicator control gives you the familiar jelly bean and the image of the person. The Source property should be bound to the user's SIP Uri. What the heck is a SIP Uri? The short version is that it is the address for initiating a communication session with a user via Session Initiation Protocol. You can read the long version here and here.
You may be asking yourself what any of this has to do with LINQ to LDAP. Odds are if you are tracking anything about your users in your applications it's either the sAMAccountName or the Guid. The SIP Uri for those users, however, is stored in the msRTCSIP-PrimaryUserAddress attribute. You can perform a simple lookup to get this attribute. Query by example will not work in this case since the property name is invalid, but mapping or dynamic querying will work.
ASP.Net isn't quite as straightforward. Here's a great post about piggybacking on the NameCtrl in SharePoint 2010. Make sure you do not prefix the uri with "sip:" in the ASP.Net example or it won't work.