DNS is my favourite hack in that respect because so few people are aware of it.
For those who don't know, there are even full IP proxies that uses DNS [1], but you can hack up a primitive one using shell script by basically setting up a nameserver for a domain, turning on all query logging and using a shell script that splits your file up, encodes it into valid DNS labels and requests [some encoded segment].[yourdomain]. Now your file will be sitting in pieces in your DNS query log and all you need is a simple script to re-assemble it.
Best of all is that it works even if it passes through intermediary DNS servers, such as a corporate proxy, unless it's heavily filtered (e.g. whitelisting domains) or too rate limited to be useful.
For those who don't know, there are even full IP proxies that uses DNS [1], but you can hack up a primitive one using shell script by basically setting up a nameserver for a domain, turning on all query logging and using a shell script that splits your file up, encodes it into valid DNS labels and requests [some encoded segment].[yourdomain]. Now your file will be sitting in pieces in your DNS query log and all you need is a simple script to re-assemble it.
Best of all is that it works even if it passes through intermediary DNS servers, such as a corporate proxy, unless it's heavily filtered (e.g. whitelisting domains) or too rate limited to be useful.