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:
- Taiwanese-Mandarin Dictionary
Created by Iuⁿ Un-gian of the National Taiwan University.
- Taiwanese Southern Min Dictionary
Produced by the Ministry of Education.
- 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 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.