sbdiff is a frontend for (GNU) diff with colorized side by side output
...It compares files and directories line by line, outputs the files side-by-side in two columns and colorizes the differences.
Because it is a console application, no GUI is necessary.
hg-diff is a simple GUI program to browse mercurial revisions. It is used to display a summary of all changes between two revisions and to display a graphical comparison of the two versions of each changed file.