Group List of Objects based on Property using Linq?

It sounds like you want something like:

// No need to sort sites first
var grouped = sites.OrderBy(x => x.Type)
                   .GroupBy(x => x.Type);

Then just serialize grouped. However, I don’t know quite what an IGrouping will look like in JSON… and the type will be present in each case. You may want something like:

var grouped = sites.OrderBy(x => x.Type)
                   .GroupBy(x => x.Type)
                   .Select(g => new { Type = g.Key,
                                      Sites = g.Select(site => new {
                                                           site.Title,
                                                           site.URL
                                                       } });

I think that would give you a nicer JSON structure.