What tool do you use for comparing two files during your development & troubleshooting actions?

Can I recommend a safe and freely available product called DiffMerge from SourceGear...

Product features

Graphically shows the changes between two files. Includes intra-line highlighting and full support for editing.

Graphically shows the changes between 3 files. Allows automatic merging (when safe to do so) and full control over editing the resulting file.

Performs a side-by-side comparison of 2 folders, showing which files are only present in one file or the other, as well as file pairs which are identical or different.

Windows Explorer Integration.

Right-click on any two files in Windows Explorer to diff them immediately.

Configurable.

Rulesets and options provide for customized appearance and behavior.

International.

Compatible with 42 different character encodings.

Cross-platform.

Identical feature set on Windows, Mac OS X, and Linux.

While you are there, check out their other flagship tools perfect for small to medium sized development teams!