And you could replace Dropbox quite trivially by getting an FTP account, mounting it locally with curlftpfs, and then using SVN or CVS on the mounted filesystem. From Windows or Mac, this FTP account could be accessed through built-in software.
yet, you jest.
python -m http.server
In a directory as a lazy way to transfer a file (especially to my phone).It is not encrypted, and I wouldn’t be that surprised if it was also vulnerable in some ways, but it sure is easy (and it is only for lan, and pretty brief).
ruby -run -e httpdYes, it works on Windows, Linux, and OS X.
All the problems with NAT traversal should be avoided in that case.
(Also, being in the same room doesn't always mean direct connection is allowed. I've seen wifi networks that forbid it.)