Is there a tool that can be used to determine the maximum IOPS on my Datastores? Due to the way the SAN is configured (poorly by the installer) it's difficult to calculate this and we want to know where the ceiling is. It looks like we have lots of headroom but we want a way to very this.
SnapOverflow
Microsoft has a tool called SQLIO which will push the IO to the limit. Just build a Windows VM with a disk on the correct data store and run SQLIO. Don't let the name fool you, it doesn't actually require SQL to be installed (it doesn't have anything to do with SQL, it was just built by the SQL team).
You could also use IOMeter, in much the same way as the SQLIO suggestion by mrdenny. Have a look at the Unofficial Storage Performance Thread @ VMware communities where you can download a pre-configured test file for use with IOMeter and compare your results to other contributors.
In addition to iometer that dadamson suggests, take a look at ATTO diskbench. It's very easy to use, and it's free. Just google for download links.