I wrote an article for SQLServerCentral entitled "Database Space Capacity Planning" that demonstrates a database and volume (disk) capacity planning solution I use in my production environment. Some highlights of the solution:

• Powershell is used to collect database and volume space information from a list of SQL Servers, which is then loaded into a consolidated reporting database
• A series of queries are used to calculate a days remaining metric i.e. the number of days until volume or database file runs out of space.
• Uses SQL Server Reporting Services (SSRS)  to provide self-service reports

See the full article for details.