Attaching SQL Server .mdb without .ldf file

If you're using SQL Server 2005:
create a database of equal size to the one you're trying to attach
shutdown the server
swap in the old mdf file
bring up the server and let the database attempt to be recovered and then go into suspect mode
put the database into emergency mode with ALTER DATABASE
run DBCC CHECKDB (dbname, REPAIR_ALLOW_DATA_LOSS) which will rebuild the log and run full repair
Your database will be available again but you'll have lost data and the data won't be transactionally consistent - see the following blog posts:
https://blogs.msdn.com/sqlserverstorageengine/archive/2006/06/15/632398.aspx
https://blogs.msdn.com/sqlserverstorageengine/archive/2006/06/16/633645.aspx
https://blogs.msdn.com/sqlserverstorageengine/archive/2006/06/18/636105.aspx
If you're on SQL Server 2000, you can still do this but you'll need to use the undocumented DBCC REBUILD_LOG at your own risk.

original post on MSDN