UNDP has a global web site where lots of jobs and short-term contracts are posted at: http://jobs.undp.org/index.cfm.
From this page, you can access jobs by clicking on the links on the left. For short-term opportunities, click on "Consultancies" on the bottom left corner; you will access the page where all short-term opportunities are listed by category - MDGs, Democratice Governance, Poverty Reduction, Environment and Energy, Crisis Prevention & Recovery, HIV/AIDS, Women's Empowerment, and Management.
In addition, you may check each UNDP Country Office web site (find their web site on Google). Some country offices are advertising their job opportunities only through their web site and are not using the global UNDP jobs web site.