This is my list of SQL Server extended properties editors I have recently tested.

ApexSQL Search

ApexSQL Search it free add-ins for SSMS and Visual Studio. The primary feature of the tool is to search for database objects, but it also enables you to edit extended properties (not only MS_Description).

DBScribe for SQL Server

DBscribe enables you to edit descriptions saved in extended properties in SQL Server. To describe object you should select it in the tree and then you can enter description in the window below. You can export documentation to HTML, CHM, MS Word, XML formats.

SQL Server Extended Properties Quick Editor

This is a simple, free tool for editing extended properties. To connect to server you should modify connection string in xml file. This tool allows you to edit any property, not only MS_Decription. It also enables you to create properties.

Redgate SQL Doc

Redgate SQL Doc is a simple tool for describing and generating documentation of SQL Server. It enables to edit field descriptions. What is slightly annoying is that you have to save each field to the database individually. It also generates database documentation in HTML format.

Dataedo

Dataedo is different kind of tool on this list. It is a powerful documentation tool metadata repository - all descriptions and extended properties are stored in separate database (a repository). You can create multiple custom fields and choose to import/export some of them from/to SQL Server properties (it also works with other databases). It has a convenient editor where you can document all table fields at once and it supports dropdowns in editor. It also generates nice HTML documentation.