JMLR – Journal of Machine Learning Research
From the webpage:
The Journal of Machine Learning Research (JMLR) provides an international forum for the electronic and paper publication of high-quality scholarly articles in all areas of machine learning. All published papers are freely available online.
Starts with volume 1 in October of 2000 and continues to the present.
Special topics that call out articles from different issues and special issues are also listed.
A first rate collection of machine learning research.