The specific gap side-by-side covers for me is reviewing changes on a remote box without firing up an IDE. Delta is great but keeps the unified format. icdiff does the split view but is pretty barebones. So there's definitely space here.

What nobody's mentioned yet is difftastic. Takes a completely different approach - parses syntax trees instead of lines, so indentation changes and bracket shuffles don't show up as noise. Worth a look if you're comparing options.

Main question I'd have: how does it hold up on large files? 5k+ line diffs are where most of these tools either choke or produce unreadable output. That'd be the test I'd run first.