Meld: winmerge on non-ms!

From the Meld homepage:

Meld is a visual diff and merge tool. It integrates especially well with CVS. The diff viewer lets you edit files in place (diffs update dynamically), and a middle column shows detailed changes and allows merges. The margins show location of changes for easy navigation, and it also features a tabbed interface that allows you to open many diffs at once. Look at the screenshots page for more detailed features.

Another alternative, would be to use a K application: KDiff3