Morfessor is a family of probabilistic machine learning methods that find morphological segmentations for words of a natural language, based solely on raw text data. After the release of the public implementations of the Morfessor Baseline and Categories-MAP methods in 2005, they have become popular as automatic tools for processing morphologically complex languages for applications such as speech recognition and machine translation. This report describes a new implementation of the Morfessor Baseline method. The new version not only fixes the main restrictions of the previous software, but also includes recent methodological extensions such as semi-supervised learning, which can make use of small amounts of manually segmented words. Experimental results for the various features of the implementation are reported for English and Finnish segmentation tasks.