Can I post from outside wordpress.com?

Yes.

You can use a program sometimes called a ‘weblog client’ to post to your blog - or blogs - here at wordpress.com

There are many such programs available and a good list to start with is at the WordPress Codex - http://codex.wordpress.org/Weblog_Client

They will all ask for your ‘endpoint’. And your endpoint is xmlrpc.php at the end of your blog name:

http://faq.wordpress.com/xmlrpc.php
Many of the clients are free and even those that are not usually have a trial period.