Within SQL Server an index is used to optimise the reading of data from the database.

However the side effect of having such an index is reduced write speed for inserts and updates. This is because SQL Server will need to update and maintain the index.

Running the below SQL will highlight indexes which have not been used recently, allowing you to remove these indexes (if these are no longer required). This is sorted by the number of updates, as tables with high number of updates will most likely benefit from removal of this index.

SELECT TOP 200 o.NAME AS ObjectName
	,i.NAME AS IndexName
	,i.index_id AS IndexID
	,dm_ius.user_seeks AS UserSeek
	,dm_ius.user_scans AS UserScans
	,dm_ius.user_lookups AS UserLookups
	,dm_ius.user_updates AS UserUpdates
	,p.TableRows
	,'DROP INDEX ' + QUOTENAME(i.NAME) + ' ON ' + QUOTENAME(s.NAME) + '.' + QUOTENAME(OBJECT_NAME(dm_ius.OBJECT_ID)) AS 'drop statement'
FROM sys.dm_db_index_usage_stats dm_ius
INNER JOIN sys.indexes i ON i.index_id = dm_ius.index_id
	AND dm_ius.OBJECT_ID = i.OBJECT_ID
INNER JOIN sys.objects o ON dm_ius.OBJECT_ID = o.OBJECT_ID
INNER JOIN sys.schemas s ON o.schema_id = s.schema_id
INNER JOIN (
	SELECT SUM(p.rows) TableRows
		,p.index_id
		,p.OBJECT_ID
	FROM sys.partitions p
	GROUP BY p.index_id
		,p.OBJECT_ID
	) p ON p.index_id = dm_ius.index_id
	AND dm_ius.OBJECT_ID = p.OBJECT_ID
WHERE OBJECTPROPERTY(dm_ius.OBJECT_ID, 'IsUserTable') = 1
	AND dm_ius.database_id = DB_ID()
	AND i.type_desc = 'nonclustered'
	AND i.is_primary_key = 0
	AND i.is_unique_constraint = 0
	AND dm_ius.user_seeks + dm_ius.user_scans + dm_ius.user_lookups = 0
ORDER BY UserUpdates DESC

The following line can also be altered in order to include indexes which are rarely used (The code above is for unused), as there may be some indexes which are expensive to maintain, but only used once a day for example!

AND dm_ius.user_seeks + dm_ius.user_scans + dm_ius.user_lookups = 0

Example of result from this is shown below: