Seems some companies began to integrate compression technologies transparently to their end-user products. One of the products which I found useful is Opera Web Browser. It uses a new feature called "Turbo". Actually it's a way to download data over Opera's proxy servers. And proxy server itself compresses images in lower qualities. Thus gains extra virtual bandwidth. It's very useful feature in especially Turkiye like countries where telecommunication costs are a bit expensive. For example, I have a limited (4GB max) 3G connection. And Opera virtually expands my connection quota limit. For example it saved ~13 MiB for a few hours web surfing.

I know, somewhat jpeg, png etc recompressions are more exciting But, IMO it's a good starting point. As a small improvement, bit stream context modeling can be applied (i.e. instead of using previous n-bytes as context use n-bits as context for next bit). This small model saves ~5% on huffman coded data (jpeg, zip etc).

BTW, here is wikipedia text about Opera's Turbo feature:
Opera Turbo is a feature intended for slower internet connections; when enabled, Opera's servers act as a proxy which compresses the requested web page by up to 80% before sending it to the user. This process reduces the total size of the data being sent, and thus shortens the amount of time necessary for the page to load. This technique is also used in Opera Mini.