for sql 2008 onwards, free space on drive + percentage

SELECT distinct(volume_mount_point),

total_bytes/1048576 as Size_in_MB,

available_bytes/1048576 as Free_in_MB,

(select ((available_bytes/1048576* 1.0)/(total_bytes/1048576* 1.0) *100)) as FreePercentage

FROM sys.master_files AS f CROSS APPLY

sys.dm_os_volume_stats(f.database_id, f.file_id)

group by volume_mount_point, total_bytes/1048576,

available_bytes/1048576 order by 1