For who does not have vCenter Server here are some features that can be realized (of course with a lot of limitations) also without this product.
Note also that now there is a new free management tool specific for ESXi that can do a lot of interesting task:
VMware Go - http://www.vmware.com/go/vmware-go/
VMware to launch Go: a free web management service for ESXi - http://www.virtualization.info/2009/08/vmware-to-launch-go-free-web-management.html
VMware Free Service Simplifies ESXi Hypervisor Use - http://www.eweek.com/c/a/Virtualization/VMware-Free-Service-Simplifies-ESXi-Hypervisor-Use-341468/
Also is possible to build it's own management functions with scripting.
But note some functions will be available ONLY with vCenter Server:
VMware HA setup (but VMware HA then can work also without vCenter)
VMware vMotion (the migrate function is available only in vCenter)
VMware Storage vMotion (the migrate function is available only in vCenter)
Host and VM Management
The client is still the same (vSphere Client or VI Client), just point directly to the ESX/ESXi name or IP.
You will loose all vCenter specific menu (migrate, template, cluster, converter, maps, alarms, ...)
VMware HA require vCenter Server only during installation, that the HA agents will work on the ESX/ESXi. Also you must be licensed for HA.
Without vCenter Server and with only two hosts (but is possible to do also with more) you have to use shell script and VMware CLI.
Also you can try Starwind: in this whitepaper (http://www.starwindsoftware.com/vmware-availability-guide) you can read how to implement HA using SAN.
http://kb.vmware.com/kb/1000936 - Moving or copying a virtual machine within a VMware environment
VM Host Migration (with shared storage)
If you have vCenter, you could shutdown the VM and do a cold migration. Or use VMotion on a live VM but only if you have VMotion license and a shared storage.
If you do not have vCenter Server, you can simply power off the VM, go on destination host, browse the datastore with the required VM, right click on the vmx file, add to the inventory and then power-on the VM.
There could be a question about keep or modify the UUID of the VM (on vSphere the question is about you have moved or you have copied the VM). Choose "keep" (or "moved").
This is very useful also in case where exist a vCenter Server, it's a VM, but there isn't a VMotion license (like in vSphere Essential bundle).
VM Storage Migration
If you have vCenter Server, you could shutdown the VM and do a cold migration. Or if the ESX host in which the guest is on local storage, can see the new shared storage, you can do an SVMotion and then you wouldn't have downtime.
If you do not have vCenter Server, you can copy the VM folder (with VM powered off) using cp commnad from ESX CLI.
Note that your datastore are under /vmfs/volumes.
After the copy you add add the "new" VM to the inventory, test it, and if all works delete the old one.
VM file renaming
If you rename the VM name, the VM files associated are not renamed. With vCenter a simple trick is make a storage migration (or Storage vMotion) to fix the name.
Without vCenter the solution is described in this thread: Rename virtaul machine in Datastore
Upgrade a major release of ESX/ESXi
With vCenter Server is very simple: just use VUM.
Without vCenter Server there are different solutions.
ESXi 3.5 -> ESXi 4.0
You can do it with "host upgrade utility" which is included to vSphere Client bundle.
http://kb.vmware.com/kb/1011542 - Take a look at this VMware kb here on a method to obtain the Host Update Utility since you don't have vCenter.
You then download the media and add the media to the host update utility...
ESX 3.5 -> ESX 4.0
http://kb.vmware.com/kb/1009440 - Performing an offline upgrade from ESX 3.x to ESX 4.0 on a local ESX host
Update to minor release
For ESX there is the esxupdate utility that can handle update.
http://kb.vmware.com/kb/1016209 - Updating ESX 4.0 to ESX 4.0 Update 1
For ESXi vSphere Client with Host Update Tools can be a good solution.
VCB and most of the beackup programs can works both with vCenter Server or directly with ESX/ESXi.
Just point to the ESX name or IP.
SRM without vCenter