I wouldn't do this if I were you, but if you like:

Download FreeBSDonUSB-6.1RELEASE-v0.01.gz, gunzip it, dd it to your USB
stick and boot it.

You should have 64MB of RAM at the least, and a system that will boot
from USB.

NOTE THAT THE SYSTEM AS DISTRIBUTED CONTAINS A NULL ROOT PASSWORD AND BY
BOOTING IT YOU ARE POTENTIALLY LEAVING YOUR PANTIES DOWN.  SO DON'T.

These files are distributed under the same terms as FreeBSD, and are:
        Copyright 2006 Ceri Davies, ceri@FreeBSD.org.

THIS SOFTWARE IS PROVIDED BY CERI DAVIES AND CONTRIBUTORS ``AS IS'' AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED.  IN NO EVENT SHALL AUTHOR OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.