I\'m building an application in ASP.NET MVC (using C#) and I would like to know how I can perform calls like curl http://www.mywebsite.com/clients_list.xml inside my controller
Try using Microsoft.Http.HttpClient. This is what your request would look like
var client = new HttpClient();
client.DefaultHeaders.Authorization = Credential.CreateBasic("username","password");
var form = new HttpUrlEncodedForm();
form.Add("status","Test tweet using Microsoft.Http.HttpClient");
var content = form.CreateHttpContent();
var resp = client.Post("http://www.twitter.com/statuses/update.xml", content);
string result = resp.Content.ReadAsString();
You can find this library and its source included in the WCF REST Starter kit Preview 2, however it can be used independently of the rest of the stuff in there.
P.S. I tested this code on my twitter account and it works.