An impressive list. However - how do you choose the right tool for your needs? "What is the best tool for ..." is usually rejected in SO (and other Stack Exchange sites). A "Stack Overflow for tools" recommendations, pros and cons would be great.
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.
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"
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!
Like for example in SSH it would talk about setting up keys, and turning off features that make it less secure (can't think of which off the top of my head)