An editor for Portal 2 Available on Steam

Owners of the PC version can create their own levels Valve today released the second beta version of Portal Authoring Tools, a set of free tools for all owners of the PC version of the game, which can be downloaded from the "Tools" interface Steam. These are the same tools used by developers for the creation of Portal 2, which enables the creation of levels for the single player or co-op, new skins for characters, sound effects and music.

Source: Portal 2 - Official Website

• Portal 2 Level Editor (11/05/2011)
• Valve releases Portal 2 level creator (11/05/2011)
• Valve releases Portal 2 authoring tools (11/05/2011)
• Portal 2 Toolset Allows Users to Be Crueler Than GLaDOS (11/05/2011)
• Portal 2 maker punts Portal 2 making pack (11/05/2011)

Portal 2 (wikipedia)  Steam (software) (homepage)  Steam (software) (wikipedia)