using (SPSite site = new SPSite(SPContext.Current.Site.Url))
        {
            using (SPWeb web = site.OpenWeb())
            {
                SPContentType documentContentType = web.AvailableContentTypes[SPBuiltInContentTypeId.Document];
            }
        }

SPContentTypeId id = new SPContentTypeId(BaseContentTypeId);
IList<SPContentType> ContentTypes = web.ContentTypes.Cast<SPContentType>().Where(c => c.Id.IsChildOf(id)).ToList();

using (SPSite site = new SPSite(SPContext.Current.Site.Url))
{
  using (SPWeb web = site.OpenWeb())
   {
     SPContentTypeCollection allContentTypes = web.ContentTypes;
                SPContentTypeCollection docContentTypes = null;
                foreach (SPContentType objContentType in allContentTypes)
                {
                    if (objContentType.Parent.Name == "Document")
                    {
                        docContentTypes.Add(objContentType);
                    }
                }
                // docContentTypes contains all content types derived from content type "Document". You can implement additional checks and add this into a drop down list.
    }
}

using System.Collections.Generic;
using Microsoft.SharePoint;
...
using (SPSite GetSite = new SPSite("http://site"))
{
    using (SPWeb TopWeb = GetSite.OpenWeb())
    {
            SPContentTypeCollection CurContentTypes = TopWeb.ContentTypes;
            List<SPContentType> SelectedTypes = new List<SPContentType>();
            foreach (SPContentType SPCT in CurContentTypes)
            {
                if (SPCT.Group == "MyGroup")
                {
                    SelectedTypes.Add(SPCT);
                }
            }
    }
}

using (SPSite siteCollection = new SPSite("put your site url here"))
        {

            using (SPWeb Web = siteCollection.OpenWeb())
            {

                SPContentTypeCollection SPCTCol = Web.ContentTypes;
                List<string> CustomTypesList = new List<string>();
                foreach (SPContentType ContentType in SPCTCol)
                {
                    if (ContentType.Group == "put your content type group name here")
                    {
                        CustomTypesList.Add(Convert.ToString(ContentType.Name));
                    }
                }
            }
        }