You'll want to download and install an actual portal server. Apache.org provides two: Pluto, which is really good for just doing testing of individual portlets, and JetSpeed, which is a full blown portal server that is ready for both testing and prime-time deployment. They're both solid.
With those downloads, you get all the required files you need to link to in order to do portlet development with Eclipse or NetBeans.
My signature links provide a variety of tutorials which take you through everything from the Hello World Portlet, to very complicated Struts and JSF portlets. Portlet Examples and Portal Server Tutorials. The site has the dual purpose of promoting my Portlet Development Book, which youmight also find helpful.
Liferay Portal would be good choice (support JSR-168 spec. & JSR 286 spec.)
Liferay Portal is the world's leading open source enterprise portal solution using the latest in Java and Web 2.0 technologies.
* Built in Content Management System (CMS) & Collaboration Suite * Out-of-the-box usablility�choose from over 60 portlets * Out-of-the-box development tools * Out-of-the-box localization to 22 languages! * Runs on all major application servers, databases and operating systems * Benchmarked among the most secure portal platforms * Business-friendly MIT License