Gives you an observable class which will emit any changes made in teamcity (e.g. build state change, project added, build configuration removed, etc).
var teamcityObservable = new TeamcityObservable("http://teamcity.foo.com", "foo", "bar");
teamcityObservable.Subscribe(e => System.Console.WriteLine(e.EventId));
var teamcity = teamcityObservable.Teamcity;
Console.WriteLine(teamcity["Some Project"]["Some Build Configuration"].State);