TFTP is most commonly used these days to remote boot small images, move firmware and configuration details. Despite its name it shares little in common with FTP other than it's primary purpose: moving files. This post will leave you with a configured and running TFTP daemon on Gentoo.
There are a number of TFTP servers in portage, for the sake of expedience we'll only look at atftpd.
# emerge atftp
Now create a root directory to serve files from and change its permissions:
# mkdir /var/tftp
# chown nobody: /var/tftp
Edit atftp's configuration file to look like this:
# nano /etc/conf.d/atftp
# Config file for tftp server
TFTPD_OPTS="--daemon --user nobody --group nobody"
Now drop the image or whatever you plan on transferring into /var/tftp. Start the server:
# /etc/init.d/atftp start
Run this to make is start on boot:
# rc-update add atftp default
And away you go!