declare @myid int
create table #temp1(f1 int)

insert into #temp1 values( 1 )
insert into #temp1 values( 2 )
insert into #temp1 values( 4 )
insert into #temp1 values( 5 )
 --insert into #temp1 values(7)
 
 --insert into #temp1 values(8)
 
insert into #temp1 values( 10 )

 /*All "free"  id*/ 
select a.f1+ 1 
from #temp1 a 
left outer join #temp1 b on (b.f1) = (a.f1+ 1 )
where b.f1 is null

 /* First "free" id */ 
select top  1  @myid = a.f1
from #temp1 a 
left outer join #temp1 b on (b.f1) = (a.f1+ 1 )
where b.f1 is null
order by a.f1

select ISNULL(@myid,  0 ) +  1 
drop table #temp1