Flurl

var url = "http://www.some-api.com"
	.AppendPathSegment("endpoint")
	.SetQueryParams(new {
		api_key = ConfigurationManager.AppSettings["SomeApiKey"],
		max_results = 20,
		q = "Don't worry, I'll get encoded!"
	});

At its core is the Url class, which is designed to work seamlessly with strings, as demonstrated with the extension method above. Creating a Url via a string extension is purly optional though; you can create one explicitly if you prefer:

var url = new Url("http://www.some-api.com").AppendPathSegment(...

A Url also converts back to a string implicitly, so you can use it directly in any method that takes a string:

var result = await new HttpClient.GetAsync(url);

Flurl also contains the handy Url.Combine method, which is basically a Path.Combine for URLs, ensuring one and only one separator character between segments:

var url = Url.Combine("http://www.foo.com/", "/too/", "/many/", "/slashes/", "too", "few");
// result: "http://www.foo.com/too/many/slashes/too/few"

###Encoding

Flurl takes care of encoding characters in URLs but takes a different approach with path segments than it does with query string values. The assumption is that query string values are highly variable (such as from user input), whereas path segments tend to be more "fixed" and may already be encoded, in which case you don't want to double-encode. Here are the rules Flurl follows:

Query string values are fully URL-encoded.
For path segments, reserved characters such as / and % are not encoded.
For path segments, illegal characters such as spaces are encoded.
For path segments, the ? character is encoded, since query strings get special treatment.

###Url API

The Url API is small, discoverable, and fairly self-explanatory. For completeness, here are all public methods and properties:

// Static method:

static string Combine(string url, params string[] segments);

// Instance methods (each with equivalent string extension):

Url AppendPathSegment(string segment);
Url AppendPathSegments(params string[] segments);
Url AppendPathSegments(IEnumerable<string> segments);
Url SetQueryParam(string name, object value);
Url SetQueryParams(object values);
Url SetQueryParams(IDictionary values);
Url RemoveQueryParam(string name);
Url RemoveQueryParams(params string[] names);
Url RemoveQueryParams(IEnumerable<string> names);

// Properties:

string Path { get; }
IDictionary<string, object> QueryParams { get; }

###Get it on NuGet

PM> Install-Package Flurl

###Credits

Thanks to Geoffrey Huntley for providing an intial portable implementation.