Simple Console Editor for OS X Terminal
There are a bunch of editors running in Terminal already installed by default. Some of them come with quite a learning curve though...


nano rather easy to use, onscreen help, limited functionality
emacs takes some time to get used to, start with the tutorial (type Ctrl-h t)
vi/vim takes some time to get used to as well, especially because it's a modal editor, start with the help document (type :help)


And of course there is also Textedit.app. Doesn't run in Terminal but is quite a versatile editor to get started.