Chessex manufactures grid maps that you can draw upon, with water-soluble markers ("overhead" pens). These are available with a variety of pre-printed patterns, so you can get those with hexes, squares, and so on.
I personally use a mixture of one of these mats, "dungeon tiles" (tiles of various shapes with terrain pictures on them, plus pre-printed grid marks), printed out sheets for some floor plans and such. Often I place a large plexi-sheet over these: it keeps the bits and pieces underneath from moving around too much, is better to roll dice and move around counters on, and I draw on the plexi with dry-erase whiteboard markers (which are not usable on Chessex's mat material) and cleaning up after these is much nicer than the water-soluable overhead markers.
Also, I use the plexi-map as a cover for a variety of boardgames I have that come with folded up card-stock maps.
To get the sheet of plexi, I just went to a local glass-supply place and gave them the measurements I wanted and they quickly cut me a sheet. You can also get plexi at some large DIY or home supply or hardware places, but I found that the prices were cheaper at the glass supply place especially when it came to just giving them a set of measurements and asking them for a precisely sized sheet.