- FOLDOC is a searchable dictionary available as a text file and offered under the GNU Free Documentation License. You can use a database tool like SSIS to convert the dictionary into a database table & validate your words against it.
- WordNet from Princeton University is unencumbered for commercial use. ObjectGraph has a SQL Server database version of it.
Free Programmable & Searchable Dictionaries
If you had to build a custom spell check functionality or something similar for your application, there are thankfully free Dictionaries in text and database format.