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).
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.
Rulesets and options provide for customized appearance and behavior.
Compatible with 42 different character encodings.
Identical feature set on Windows, Mac OS X, and Linux