Yap is a command-line (terminal mode) Twitter client for Unix. It is not as fully-featured as many clients, but it's not bad.
Yap is written in Python and requires python-twitter and simplejson, both available via PyPI and easy_install.
Yap no longer works due to Twitter's move to OAuth. Sooner or later I will get around to fixing this. If you want sooner, file a bug report. :)