Anything. You can post anything you see fit, so long as you respect copyright. Be creative. What you post is a decision of yours.

For example, you can post advertisements of conferences and jobs. You can upload preprints of your papers, a practice permitted by most journals. You can even post a preprint of a paper already published in a journal, so that your paper gets one more chance to find its readers. You can post your lecture notes. You can express your admirations for another mechanician, or opinions concerning our field. You can also share tips with fellow mechanicians on how to make our working life easier and more enjoyable.