Sharepoint 2010 has a feature called Managed Metadata which is used for defining corporate wide Taxonomy. Managed Metadata is hierarchy of terms which once defined can be added as a tag to any item within Sharepoint 2010.
For defining and managing metadata terms Sharepoint 2010 has a tool called Term Store Management Tool. This tool is accessed from Site Administration Section of Site Settings page
The Term Store Management Tool has options for creating and managing Groups, Term Sets and terms. While this tool is simple enough to use, creating large number of terms will be cumbersome. For creating large number of terms Sharepoint provides a csv file import option. The csv file should adhere to a specific format details of which can be found here
Once the terms have been imported the Term Store Management tool can used to maintain them.
These meta data terms can then be used to tag any document or list item within the Sharepoint site. For instance to tag a document with metadata term select the document from document library and click Tags & Notes option from the Ribbon This will open the Tags & Notes window where you can enter the metadata term related to the document.
Layer2 has created a predefined taxonomy metadata which can be imported into Sharepoint 2010. More details about Layer2 predefined taxonomy metadata can be found in http://www.layer2.de/EN/PRODUCTS/Pages/SharePoint-2010-Taxonomy-Metadata.aspx