declare @ihaveatablelikethat table (RunId int, RunDateTime datetime, [RowCount] int)
insert @ihaveatablelikethat  
      select 1, '2013-03-21 10:00:00', 20
union select 2, '2013-03-21 10:15:00', 30
union select 3, '2013-03-21 10:30:00', 20
union select 4, '2013-03-21 10:45:00', 25
union select 5, '2013-03-21 11:00:00', 15
union select 6, '2013-03-21 11:30:00', 20
union select 7, '2013-03-21 11:45:00', 25
union select 8, '2013-03-21 12:00:00', 15


select DATEADD(HOUR,DATEPART(HOUR,RunDateTime),CAST(CAST(RunDateTime AS DATE) AS DATETIME)) DHFrom
      ,DATEADD(HOUR,DATEPART(HOUR,RunDateTime)+1,CAST(CAST(RunDateTime AS DATE) AS DATETIME)) DHTo
      ,AVG([RowCount])
from   @ihaveatablelikethat
group by  CAST(RunDateTime AS DATE), DATEPART(HOUR,RunDateTime) 

SELECT
    DATEADD(HOUR, DATEDIFF(HOUR, 0, RunDateTime), 0) AS AvgHourlyDateTime,
    --DATEADD(MILLISECOND, -3, DATEADD(HOUR, DATEDIFF(HOUR, 0, RunDateTime) + 1, 0)) AS AvgHourlyEndDateTime,
    AVG([RowCount]) AS AbgRowCount
FROM @ihaveatablelikethat
GROUP BY
    DATEADD(HOUR, DATEDIFF(HOUR, 0, RunDateTime), 0)
    --,DATEADD(MILLISECOND, -3, DATEADD(HOUR, DATEDIFF(HOUR, 0, RunDateTime) + 1, 0))