For people that can't be bothered to keep proftpd running all the time and are paranoid enough to disable password authentication in the OpenSSH server, but still would want to send some files to/from the N900 outside their caves: Dump FTP Server.

How it's different from proftpd? You can start it from the menu, generate one-time password, send files, then just close it like any other application. Quick, simple, doesn't leave any holes in your phone security.

Caveats:
- deb package needs to be installed by hand for now. It will be in extras in future, hopefully.
- QT 4.6 is required, which is only in extras-devel, so if you're not comfortable with enabling it, even for a short period, don't bother.
- There are bugs. For example files with spaces are handled incorrectly.
- It's ported from Symbian, and not yet fine-tuned for N900. Transfer speed is comparable to copying over ssh, but doesn't hog the CPU.