ALNAP
ALNAP is the Active Learning Network for Accountability and Performance in Humanitarian Action. Members of the Network are many of the key humanitarian organisations from across the humanitarian sector. It publishes (the very few) openings in the humanitarian sector of its Member Organisations.
Languages: EN
http://www.odi.org.uk/ALNAP/job_opportunities.htm

FAO
FAO is the Food and Agriculture Organization of he UN. It publishes both internal vacancies (called professional vacancies) and vacancies for consultants (called project vacancies).
Languages: EN CH SP FR
http://www.fao.org/VA/projva_en.htm (project vacancies)
http://www.fao.org/VA/vac_en.htm (professional vacancies)

IFAD
The International Fund for Agricultural Development maintains a page where are published both internal vacancies and opportunities for consultants (firms are preferred to individual consultants).
Languages: EN
http://www.ifad.org/job/index.htm

IFC
IFC is the International Finance Corporation (WB Group).
Procedures for the hiring of consultants are unclear and opaque; however, in the following page they publish some vacancies, which seem more internal ones rather than opportunities for consultants.
Languages: EN
http://www.ifc.org/ifcext/careers.nsf/Content/CurrentOpportunities

IMF – IEO
This is the page of the Independent Evaluation Office of the International Monetary Fund. Vacancies both for internal and contract-based vacancies are posted here. Spontaneous candidatures are also encouraged.
Languages: EN
http://www.ieo-imf.org/about/work.html

ITC - International Trade Centre
ITC is the technical cooperation agency of UNCTAD and WTO for operational, enterprise-oriented aspects of trade. Very few openings for evaluators, if any.
Languages: EN FR
http://www.intracen.org/appli1/jobman/vacancies.aspx

UNCCD
This is the page of job postings of the UN Convention to Combat Desertification.
Languages: EN FR ES
http://www.unccd.int/secretariat/menu.php?newch=l7

UNCTAD
The UN Conference on Trade and Development have a page of vacancies; yet another page of general vacancies, meaning that opportunities in the evaluation sector should go here.
Languages: EN FR ES
http://www.unctad.org/Templates/Page.asp?intItemID=2105&lang=1

UNDP
This page of the UN Development Programme publishes a large number of openings in the external cooperation field, including in the evaluation sector. Applications shall be done online, and UNDP contacts back the pre-selected candidates. Unfortunately, this is an organisation that manages the recruitment process in an opaque way, and non-selected applicants are never informed about the reasons of non acceptance of their candidature.
Languages: EN FR ES
http://jobs.undp.org/cj_view_jobs.cfm

UNESCO
UNESCO (the UN Education, Scientific, and Cultural Organization) has here a website devoted to recruitment of both internal staff and consultants. Application is online. From time to time, you can find openings for evaluators.
Languages: EN FR
http://recrutweb.unesco.org/postes/postes_visualisation.asp?AffLangue=gb&CATPOSTE=1

UNFCCC
UNFCCC is the UN Framework Convention on Climate Change; in this page they publish both internal vacancies and opportunities for consultants.
Languages: EN FR ES
http://unfccc.int/secretariat/employment/vacancies/items/1216.php

UN-HABITAT
This is the vacancy page of the UN Human Settlements Programme. Both internal vacancies and opportunities for consultants are published here.
Languages: EN
http://www.unhabitat.org/list.asp?typeid=12&catid=435

UNICEF
Likewise for other UN agencies, also UNICEF hosts a page where vacancies are published. This page needs to be monitored further to understand whether consultant opportunities are published here, as well. In another page of their website UNICEF mentions that consultants are selected from within their roster, but it is not said what steps shall be undertaken as to be included on that roster.
Languages: EN FR ES AR CH
http://www.unicef.org/about/employ/index_currentvacancies.html

UNIDO
This is the webpage for recruitment of Field Project Experts and Consultants of UNIDO, the UN Industrial Development Organization. Also opportunities for evaluators shall be advertised here.
Languages: EN
http://www.unido.org/doc/5188

UNIFEM
UNIFEM is the UN Development Fund for Women; from time to time, it publishes vacancies on this page of its website.
Languages: EN
http://www.unifem.sk/index.cfm?module=articles&page=Article&Category=50&StartRow=1&MaxRows=5

UNODC
The UN Office for Drugs and Crime lists here vacancies for staff, for temporary staff, and project vacancies. Opportunities for evaluators can fall under any of these categories.
Languages: EN
http://www.unodc.org/unodc/en/current_vacancies.html#pv

UNOPS
UNOPS is the UN Office for Project Services. They publish several opportunities worldwide, and some of them are for evaluators. Hint: do not use an updating monitoring tool on this page, as vacancies are broken down across several pages.
Languages: EN
(web address too long; click here to go to the page)

WORLD BANK
This is the homepage of the service eConsult of the WB, where a large number of the openings of the WB Group are published. To access the service one needs registering (and to read carefully the Guides for consultants available on the same page).
Languages: EN
(web address too long; click here to go to the page)