What software do you use to provide your customers with VPSs and why?

Anybody? :(

That depends on your customer's need. If customer prefer Cpanel, just provide it. If the customer prefers Plesk, provide him Plesk CP. ;)

Is that what you mean?

Nono, not at all :)

I was talking in terms of virtualization software. Virtuozzo vs. OpenVZ vs. Xen, etc.

Every virtualization softwares have their own advantages and disadvantages. I would prefer VZ/OpenVZ because of its comparitively easier administration. Also, you can get much help from Swsoft as well.

OpenVZ is a free software, but if you want your clients to have a panel to do VE reboot etc. you have to buy HyperVM licence, or write your own custom scripts for that.