I discovered toonel.net shortly after I started traveling to places with crippled internet connectivity. Toonel.net's website says that it is "an experiment in a technique for conserving network bandwidth" -- but it doesn't mention how useful the experiment actually is! When I need to conserve network bandwidth, I use toonel for all http (web) traffic as well as IMAP and SMTP connections. I've had really good luck with web compression. Alongside standard text compression, toonel has the option to recompress web images. I currently have JPG images set to be recompressed to a quality level of 90 (a noticeable degradation in image quality -- but it can save as much as 50% in traffic). At the moment, I'm averaging around 38% outbound and 37% inbound compression. Not bad.
Success with IMAP and SMTP has been less than perfect; I frequently get an unresponsive proxy when I connect to check mail. Still, when it does connect, compression is good.
Toonel is easy to use -- you just download the java proxy client, run it, and set up port mappings and web image compression settings. The toonel website has online tutorials for how to do this on various platforms.
I've also been investigating e-mail solutions for satellite connectivity, since traditional email protocols require too many network round-trips to be used in high-latency connections. XGate email gateway looks to be a good solution. Anyone out there used it before? :)