While I've tried many other banner ad filter proxies, such as
Junkbuster,
WebWasher, and
AdSubtract, Proxomitron has a feature that the others don't. You can use a rather sophisticated form of pattern matching to rewrite html pages on-the-fly. This has some subtle uses, from mild cosmetic changes to something approaching real hacking.
To begin with, any html tag can be altered. Don't like any of the choices for color schemes on E2? Make your own. Want to change default parameters for cgi, perl or php scripts? Set up a filter that will replace them in every link on a server. For example, you could, with a filter, replace the lastnode_id=xxxx parameter here on E2; then, as you drift around the database, you can continously soft-link to whatever lastnode_id that you choose, instead of the real lastnode_id.
Similarly, many java applets (such as chat programs, java telnet apps, and even some games) have parameters that are passed to it within the source of the page it is embedded in. Using proxomitron, you can change these parameters, enabling hidden features, spoof identities, or, in the case of the telnet apps, change what host to connect to (though why you would need to is beyond me.)