Compare two Microsoft Access databases

Below are three free products that might help.

Free Database Compare 2.0 :

The StarInix Database Compare application will allow you to compare the structure of two databases and display all the differences between them. Supported databases include: SQL Server (All Editions), MySQL and Access databases. You can compare two different databases, like access and SQL 2000. It will also allow you to create snapshots of a database and compare a live database to an snapshot. A database structure viewer is included.

Compare'EM Lite

A very simple to use utility to analyze a pair of related Microsoft Access databases, determine what has changed from one to the other, and generate VBA code to propagate those changes.

Total Access Analyzer

Total Access Analyzer examines all your database objects to provide extensive documentation, cross-reference, and application and flow diagrams, with over 350 presentation-quality reports.

EDIT : Please remember that this answer dates from 2009 before downvoting it.

Quite late but: with Office 2013 Professional Plus or Office 365 ProPlus, Microsoft now offers a Database compare tool.

Edit:
One could also use the msaccess-vcs-integration add-in to export everything to text, then use any text comparison tool, or a versioning tool like Git.

Try the free version of AccDBMerge.

Be aware of the following restrictions though:

AccdbMerge Free provides basic data diff and merge facilities with the following limitations:

• both tables must have compatible primary keys,
• no more than 50 fields in each table,
• no multivalue/attachment/binary fields.

Pro version does not have these limitations. Also, it allows to compare data between files with different security options.