Splinter is a light-weight addition to Bugzilla that allows a patch reviewer to view a nicely formatted version of a patch in their web browser and add comments on particular sections.

Subsequent reviewers or the original patch author can then view the comments inline in the Splinter display of the patch.

Reviews are saved to the bug as specially formatted comments that combine small amounts of unified-diff like output with the review comments. The output is designed to be both machine-parseable and also meaningful to someone who is viewing it straight without using Splinter.

Browse Git Repository (clone git://git.fishsoup.net/splinter)
User documentation
Blog entry introducing project
Blog entry discussing Review Board vs. Bugzilla