General Portals include: OpenDirectory, Google, Yahoo, Lycos, MSN and many more. These have a bad reputation for degenerating into pages of ads (Yahoo being a prime example), becase they are usually built and maintained by a single company in an effort to direct users (who possibly use the company as an ISP and have the portal as a default homepage) towards commercial clients of the company. Attempts to do this are of limited success due to the NetworkEffect.
Subject specific portals include: FirstGov (a portal for the American Government), Eco-Portal (an environmental portal) and Alfy (a childrens portal). Generally the narrower the target demographic and purpose, the clearer the defining vision and the better the portal. Many school and university libraries have portals which aim to provide relavent links to course materials, research materials and information that students need.