VMWare does a pretty poor job providing a complete list of direct download links to the different versions of the VMWare vSphere clients. Here is a complete list of all vSphere clients from 4.1 onward.
In most cases you can install multiple client versions at once if you need to connect to multiple versions of VMWare ESXi/vCenter. The clients in the same major version are also backwards compatible, so you can for example install 5.5 update 3 and be able to manage a server with 5.5 update 1. When you launch the vSphere client and connect to a host or vCenter server it will automatically use the appropriate version that is installed on your system.
Update: VMWare now provides a list, but it does not include 4.x so I will continue to update this one. It’s also a lot easier for me to refer to this one than to dig it up from vmware/google all the time.