Water that repeatedly collects on the floor beneath a toilet or around its base can indicate some type of leak or may be caused by condensation from a toilet that sweats. For more about condensation, see Toilet Sweats.
Toilet leaks can come from a few different problems, including cracks in the porcelain, leaking seals, or a defective wax ring that seals the bottom of the toilet to the waste pipe. For information about leaks that are inside the toilet (typically from the tank to the bowl), please see How to Repair a Running Toilet.
If water is dripping down onto the floor, you can usually identify the source by mopping up any wetness, spreading out a dry newspaper under the toilet, and watching for wet spots. Use a flashlight to check the fittings directly above the wet spots for drips. In some cases, all you need to do is tighten a fitting slightly. If that doesn't do the job, you may need to replace the rubber washers or seals at the connection points. Be sure to check the water supply tube both at the valve and where it attaches to the tank--this is a common source of leaks.
If water seeps out from under the toilet's base, either the toilet needs to be tightened down to the floor or--more likely--the wax ring that seals the connection between the toilet's base and the waste pipe (closet flange) needs to be replaced. If the toilet feels loose against the floor, pry the plastic caps off of the anchor bolts, and slightly tighten the nuts that hold the toilet down. Note: Be careful not to over-tighten because this can crack the porcelain base! If the toilet doesn't feel loose, remove the toilet and replace the wax ring, as discussed in How to Replace a Toilet's Wax Ring.
Before you pull the toilet to replace the wax ring, check for cracks in the tank and bowl. If either the toilet tank or bowl are cracked, you will need to replace the toilet. For information on how to do this, see How to Install a Toilet.