Sandbags are common tools for studio photographers to weigh down studio lights and keep them from tipping over.
I have a Kinesis Safari Sack beanbag that serves as a long lens support and that also works as tripod balast. I often hang my camera bag from my tripod to provide ballast. If it's gusty, ballast can help dampen vibrations. My Gitzo Explorer tripod has a hook on the center column for this purpose (and it can keep your camera bag out of the mud).
Another solution is the 3146 Bogen "tripod utility apron". This attaches to the legs at mid height and you place in it whatever at hand, a rock, some other camera gear, etc. I may get one of these soon.