Most of the useful sites out there concerned with Taiwanese are naturally written either in Hàn-lô or in Mandarin Chinese. Those links here which require literacy in Chinese are marked with this symbol: 
Dictionaries
- Taiwanese-Mandarin Dictionary
Created by Iuⁿ Un-gian of the National Taiwan University. - Taiwanese Southern Min Dictionary
Produced by the Ministry of Education.
Language Learning
- Tâi-ôan Chè-chō
A new phrase every day, written in POJ. By Matthew O'Connor. - Intermediate Taiwanese Grammar
Also by Matthew O'Connor, this blog models sentence patterns in Taiwanese. - Maryknoll Language Service Center
With locations in both Tâi-pak and Tâi-tiong, these schools opened to cater to Catholic missionary students, but now accept non-missionary students too. - Taipei Language Institute
School with nine locations in Taiwan, China and the United States. Not all offer Taiwanese instruction, so check the website for details. - Hokkien (Minnan) language forum
Discussion, in English, of Southern Min dialects. - Washington DC Taiwanese School
Based at the TECRO Cultural Center in the US capital, the school offers weekend courses in Taiwanese for both children and adults.
Wikipedia articles
Wikipedia is user-generated content, so don't take everything you read there for absolute truth (check for well-referenced pieces as they are generally more reliable). I am slowly improving articles related to Taiwanese, correcting them, and adding proper references as I have time. The Pe̍h-ōe-jī article has already been given a good reworking.
There is also a version of the encylopaedia written in Pe̍h-ōe-jī.
General information about Taiwan
- Forumosa
Long-standing forum site for English speakers. - Taiwanderful
Articles on Taiwanese travel, events and history. - Taiwanease
Multi-featured Taiwan information site with a directory, articles, and forums.
