Howto: Compare two files using freeware or open source
Diffmerge is my favorite program (open source at least) that I use to compare files.
From their website:
DiffMerge is an application to visually compare and merge files for Windows, Mac OS X and Linux.
DiffMerge in Vault
DiffMerge was developed as the internal compare/merge tool for SourceGear Vault.
Vault is a version control tool designed to be a compelling replacement for Visual SourceSafeand offers optional, integrated bug tracking.
Vault includes DiffMerge, and is available for immediate download; installs in minutes; and is FREE to try for 30 days (and always free for single users).
Learn more about Vault.
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