var filteredFiles = Directory
.GetFiles(path, "*.*")
.Where(file => file.ToLower().EndsWith("aspx") || file.ToLower().EndsWith("ascx"))
.ToList();
Edit 2014-07-23
You can do this in .NET 4.5 for a faster enumeration:
var filteredFiles = Directory
.EnumerateFiles(path) //<--- .NET 4.5
.Where(file => file.ToLower().EndsWith("aspx") || file.ToLower().EndsWith("ascx"))
.ToList();