For Silverlight there is a library called Xceed which amongst other things "Lets you compress data as it is being uploaded.", it is not free though. I believe that this can only be done via a technology such as Flash or Silverlight and not natively on the browser.
I disagree with the above poster about browsers doing this automatically and I believe this only happens with standard HTML/CSS/Text files and only if the server and browser both have compression enabled (gzip, deflate).
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
