A Script to insert dummy Data in all tables of Database
There are lots of cases that @AmmarR's solution doesn't handle - ROWVERSION/TIMESTAMP, computed columns, columns with check constraints, foreign keys, UNIQUEIDENTIFIER columns that default to NEWSEQUENTIALID(), date/time columns that default to GETDATE(), sparse columns, etc. etc.

Why reinvent the wheel? There are tools out there that can generate data for you and they've probably already thought about all of these restrictions. For example DTM Data Generator (not free, have not tested it) and Red-Gate SQL Data Generator (not free, have tested it and recommend it - also reviewed here).