Under virtualization: XenServer -- it has Xen, but most SysAdmins wont use raw Xen, but instead a complete hypervisor such as XenServer or VMware, etc.
XenServer was recently re-rereleased and now is 100% open source (all features). Now you get all the "enterprise" features in the freebie Open Source version.
Under SMTP Servers: Zimbra - excellent top-of-class email server, and open source.
Everyone else feel free to join in on the discussion (i'm probably breaking all kinds of site rules, aren't I?:) It's a greenfiield setup so I'd like to start with what the folks around here might recommend.
If one of the choices is correct by a very margin, let me know that's the case and I'll go checkout that path. Thanks!
Zimbra is a groupware suite with Postfix under the hood. It doesn't really belong in that category. (Agreed though... it's fantastic!)
Zimbra is added under "Collaborative Software"