Tap water is a no-no due to the high mineral content. It could clog up your core real bad in time. If this is all you have available, then I'd change it out on a monthly basis at minimum. Filtered tap water is really no better.
Coolants will work but not to the degree that one would think. They tend to be thicker and make the pump work more. Also, some pump manufacturers are not to keen on keeping a warranty on a pump in which coolant has been used.
To a great extent, distilled water is best. In tests I believe it either came in as the best option but if not, then the second best option by very little margin. It's inexpensive (US$1 per gallon, gallon will last a looong time), mineral free and crystal clear with no sedimentation. Use DW along with either a nice silver kill coil (+/- US$5) or get some really inexpensive PT-Nuke and you will be set. You will not run into the "green-water" issues and such. Change every 6-12 months on average.
In case you'd like to read up[ some more on this, here are some reference articles:
Water Coolant Chemistry Part 1
Water Coolant Chemistry Part 2
Results of a Coolant Shootout
(Not a comprehensive listing)Edited by Kahbrohn - 5/9/11 at 11:41am