Pre-compiled binaries for GNU Radio
For most platforms, there are binaries available which saves you having to compile GNU Radio yourself.
Make sure you're using a not-tool-old version!
Linux standard distro packages
Some Linux distributions (Debian, Ubuntu and Fedora) have binaries available through their packaging system.
If possible, this is the easiest way to install binaries, e.g. through
$ sudo apt-get install gnuradio
$ sudo dnf install gnuradio
If you're doing this, make sure you're using a fairly recent version. You should be at least on the latest minor, e.g. if the most current version is 3.7.2, installing a 3.7.0 is OK, but 3.6.5 is too old. In that case, find another source for your binaries.
Mac OS X
There are no pre-built binaries of GNU Radio or UHD for Mac OS X at this time (early 2014). That said, installing them is quite simple; refer to the Mac OS X install guide.
Alexandru Csete has a PPA for those running Ubuntu 12.04 on a Beaglebone, Parallella or other ARM device with FPU: