Where is Microsoft.Sharepoint.IdentityModel.dll

These answers are correct if you're using SharePoint 2010.

If you need to find the DLL for SharePoint 2013, you'll need to reference it from this location: C:\Windows\Microsoft.NET\assembly\GAC_MSIL\Microsoft.SharePoint.IdentityModel\v4.0_15.0.0.0__71e9bce111e9429c\Microsoft.SharePoint.IdentityModel.dll

Note that under .NET 4.0, the location of the GAC has changed.

1. start Command Prompt

2. write "cd :\Windows\assembly\GAC_MSIL\Microsoft.SharePoint.IdentityModel\14.0.0.0__71e9bce111e9429c\"

3. write "copy Microsoft.SharePoint.IdentityModel.dll c:\"

4. now you can access the dll freely (e.g. copy to TFS, open with reflector or any other action)

Another option is to write "subst i: %windir%\assembly\gac_msil" in Command Prompt, that way you can browse the GAC_MSIL (where Microsoft.SharePoint.IdentityModel is located) via Windows Explorer.

for SharePoint 2013 C:\Windows\Microsoft.NET\assembly\GAC_MSIL\Microsoft.SharePoint.IdentityModel\v4.0_15.0.0.0__71e9bce111e9429c