Security for list item - How many way to deal with permission on item level?

Scenario: There is a library contain documents, which same type. But we have three departments using this library. We must do how each departments not see documents from other. For example: we have documents for Hr, Marketing, Administration. The document is categorized by a Field.
=> What is the best solution for this scenario when using SharePoint?

Here is the Solution  - https://www.nothingbutsharepoint.com/sites/eusp/Pages/SharePoint-Easy-Row-Level-Security.aspx