Patacriticism has just released Juxta 1.0:
Juxta is an open-source cross-platform tool for comparing and collating multiple witnesses to a single textual work. The software allows users to set any of the witnesses as the base text, to add or remove witness texts, to switch the base text at will, and to annotate Juxta-revealed comparisons and save the results.
Juxta comes with several kinds of analytic visualizations. The primary collation gives a split frame comparison of a base text with a witness text, along with a display of the digital images from which the base text is derived. Juxta displays a heat map of all textual variants and allows the user to locate — at the level of any textual unit — all witness variations from the base text. A histogram of Juxta collations is particularly useful for long documents. This visualization displays the density of all variation from the base text and serves as a useful finding aid for specific variants. Juxta can also output a lemmatized schedule (in HTML format) of the textual variants in any set of comparisons.
… The document comparison algorithm implemented in Juxta is a modified version of the Java port of Diff by Ian F. Darwin.
(Hat tip Geoff Rockwell)