I'd recommend floorplanner.com for modern games. It's a quick and easy way to draw up basic...um...floorplans for your game. If I were using it for an online game, I'd probably import it into maptool or something like that so I could layer it and add tokens and stuff.
I like pyromancers.com. It's a little clunky at first, but you can quickly produce cool looking dungeon and outdoor maps, either grid or hex, and with coordinates marked.
"Most people aren't really brave - they're just too stupid to know better. Maybe you're different, but it's doubtful." - Dungeons the Dragoning 40,000 7th Edition