For Linux:

If you want a free control panel, then Webmin/Usermin is probably near enough your only decent option.

If you're prepared to pay for a control panel, I'd take a look at Plesk and DirectAdmin.

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Damien Ransome
layershift :: UK & USA-based Managed Dedicated, DDS, VPS, Reseller & Shared Hosting
Experienced SWsoft Platinum Partners (Plesk since 2001, Virtuozzo since 2004)