Looking through the existing portfiles, I see gettext which declares it will work on linux, libpng says it will work on linux, freebsd and sunos, and monit claims to work on linux, freebsd, netbsd, openbsd and solaris. In addition to Mac OS X, of course. And, as I say, many ports will probably work on your other OSes, even if they don't declare it. (And if you find that they do work, you can tell us so that we can indicate that the port does work on that platform.)
Probably need to re-inventory these, since some of those flags are leftovers from DarwinPorts and not all ports no longer work on systems that they claim ? Feel free to post results on porting base or ports over to other operating systems on the macports-dev list, I am trying to run it on FreeBSD myself.
As long as Tcl and GNUstep are installed, and some minor bugs patched, MacPorts 1.5 installs OK on FreeBSD and Fedora. So there is no reason why there couldn't be packages (TBZ or RPM) made for those systems made in e.g. MacPorts 1.6, assuming that there is enough interest in such ports of course. "selfupdate" doesn't work, though. The issues are reported on Trac, under the keywords "FreeBSD support" and "Linux support". --anders