Since the release of ESXi 5.0 on August 24th 2011 VMware and 3rd party hardware vendors have updated and added lots of device drivers that can be downloaded in the Drivers & Tools section of VMware's vSphere 5 download center. These drivers are not included in the original ESXi 5.0 installation ISO.
However, at the same place you can download something that is called the vSphere Hypervisor ESXi 5.0 Driver Rollup 2. In fact this is the original installation ISO plus all drivers that were updated or added up to the release date of the rollup (for the Rollup 2 this is Feb 8 2012). So, if you are going to install a new server you should choose this ISO (instead of the GA release ISO) to make use of the latest drivers.
There have been reports that you cannot download the Rollup ISO if you have the free Hypervisor license only, but this is not true (at least not anymore): If you have registered for the Free VMware vSphere Hypervisor you will also be able to download the Rollup ISO.
Please be aware that the Rollup ISO does only contain updated drivers, but no Hypervisor patches, so after deploying a machine with it you still need to apply the latest VMware patches in order to have a system that is fully up to date. There are instructions available on how to do this if you do not have vCenter Update Manager available: See e.g. a post by Chris Colotti.
Update (2012-03-15): There is an issue with the current Rollup 2 ISO that you need to be aware of, especially if you are considering the installation of VMware vShield 5.0. Read more in this blog post.