I previously reported my successful installation of FreeBSD on a Soekris net4801. While the Soekris is a really popular small form factor system, it lacks a fan to keep moving components (like laptop HDDs) cool. It's also not the sort of system you can use to replace a tower PC, since it doesn't have video output, a CD, or mouse and keyboard inputs.
If you need the sort of functionality a true PC provides, but want small form factor, check out Padova Technologies. I just installed FreeBSD 5.3-BETA6 on their SlickNode Mini PC. You can see my dmesg output at the NYCBUG dmesg archive. The box is equipped with two NICs -- one is an Intel NIC (fxp0) and the other is unfortunately a Realtek NIC (re0). When you order a SlickNode you can opt for a quad NIC to be installed, or a Wi-Fi card, or several other options. This is a great appliance box for systems that need more of a PC's functionality.