String name = 'mysite';

Site site = [
    SELECT GuestUserId
    FROM Site
    WHERE Name = :name
];

String path = '/sites/servlet.SiteDebugMode';
PageReference pr = new PageReference(path);
pr.getParameters().put('guid', site.GuestUserId);
pr.getParameters().put('sitedebugmode', 'x');
String url = pr.getContent().toString().substringAfter('URL=').substringBefore(path);

System.debug(url); //eg "http://dev-bigass.cs81.force.com/mysite"

Site site = [SELECT GuestUserId, Name, Subdomain, UrlPathPrefix FROM Site WHERE Name = :'My Site'];

    Organization org = [SELECT InstanceName, IsSandbox, OrganizationType FROM Organization];

    String siteFullUrl = 'https://' + site.Subdomain + '.';
    siteFullUrl += (org.IsSandbox || org.OrganizationType == 'Developer Edition' ? (org.InstanceName.toLowerCase() + '.') : '') + 'force.com';
    siteFullUrl += '/' + site.UrlPathPrefix;

Site s = [SELECT s.Subdomain FROM Site s LIMIT 1 ];
String urlCode = String.valueof(URL.getSalesforceBaseUrl().toExternalForm());
String [] cuts = urlCode.split('[.]');
urlCode = '';
for(Integer i = 1; i < cuts.size(); i++){
    if(!cuts[i].contains('visual')){
        urlCode += cuts[i] + '.';
    }
}
urlCode = urlCode.substring(0, urlCode.length()-1);
urlCode = 'https://'+s.Subdomain+'.'+urlCode+'/yourpage;

ApexPages.currentPage().getHeaders().get('Host');

ApexPages.currentPage().getHeaders().getUrl();

String url = 'https://' + ApexPages.currentPage().getHeaders().get('Host') + ApexPages.currentPage().getHeaders().getUrl();

String url = 'https://' + ApexPages.currentPage().getHeaders().get('Host') + '/' + acc.Id;

String sfdcBaseURL = URL.getSalesforceBaseUrl().toExternalForm();

apexpages.currentpage().getURL() and apexpages.currentpages().getHeaders().get('HOST')

Site mySite = [select Id from Site where Name = 'MySite'];
SiteDetail mySiteDetail = [select SecureURL from SiteDetail where DurableId = :mySite.Id];
System.debug(mySiteDetail.SecureURL);