For a general guide to Portuguese internet sites, see my Ciberportuguês page http://users.ox.ac.uk/~srp/ciberportugues.html, and the resource lists on the Linguateca site. This document will be placed on my website with links to all the sites mentioned.
1. General sites of linguistic interest
The Linguateca site for computational linguistics http://www.linguateca.pt has extensive resource lists and provides direct access to a large number of corpora of modern Portuguese and Brazilian (see below)
The Centro de Linguistica da Universidade de Lisboa http://www.clul.ul.pt has a number of online resources for linguistics and language learning, including the Portuguese Corpus de Referência (one of the sources for the recent Dicionário da Academia de Ciências) a frequency dictionary, and samples of a collocations dictionary (dicionário de combinatórias)
The Portuguese Language page of Cambridge University http://www.cus.cam.ac.uk/~cjp16/portlang/portlang.htm has some useful links
The best collection of corpora is available through the Linguateca site http://www.linguateca.pt/
For instructions on how to use these corpora, see my Corpus searching guide http://users.ox.ac.uk/~srp/corpussearching.html
The Portuguese Corpus de Referência (one of the sources for the recent Dicionário da Academia de Ciências is accessible through the Centro de Linguística da Universidade de Lisboa http://www.clul.ul.pt
The Centro de Linguística da Universidade Nova de Lisboa has a Corpus Informatizado do Português Medieval http://cipm.fcsh.unl.pt/ (with a verb dictionary and online texts: for full access you need to request a username and password)
3. History of Portuguese
The Universidade Federal do Rio Grande do Norte hosts a simple site on the History of Portuguese (mainly cribbed from standard sources) http://www.linguaportuguesa.ufrn.br/
Similar materials are found on the Instituto Camões site
The Portuguese Language page of the Orbilat site http://www.orbilat.com/Modern_Romance/Ibero-Romance/Portuguese/index.html
has some good graphic materials and crudely translated text.
Samples of Dieter Messnerºs Dicionário dos dicionários portugueses are online at http://www.sbg.ac.at/rom/people/prof/messner/dddport.htm
5. Other electronic materials
The CLUL has issued a set of 4 CDs with samples of Português falado (available in the Portuguese Centre).