Ebuilds

First, look at the ebuild howto from the  gentoo dev handbook

Before losing your time, check  http://bugs.gentoo.org to see if the package you want to make is not already existing

You can take this one as a model : /usr/portage/skel.ebuild or go in the portage/trunk and pick one as a sample.

Create a dir for the portage overlay : mkdir /usr/local/portage

And put it in your /etc/make.conf file : PORTDIR_OVERLAY="/usr/local/portage"

Once done, you have a new, custom portage tree that will not be deleted when you sync.
You have to respect the same tree as in the gentoo portage architecture like if you want to add a tool for network analysing, you have to put it in the right folder :

/usr/local/portage/net-analyzer/foo/

For the ebuild making, please first open a bug in the ticket system, name it like the tool you want to add and upload the files in it. You may set the bug as resolved when other users have tested it.
Of course, I'm open to any help provided, so if you would like to contribute to the portage tree, catch me on IRC and let me know so I'll create a svn account for you.