Here's an old mainstay that dates back to when I was a much younger DM. I had some time to kill earlier today and figured that I'd put it to good use tying together a new set of spell and breath weapon "area of effect" templates from the kind of colorful pipe cleaner that you can find at any craft store:
They're handy things to have around, particularly if you like tabletop battles, and extra particularly if you like battles involving dragons or magic-users. The above templates are, starting with the upper left:
The white square in the middle is for 10'×10' effects like web; the red 20'×20' square for bless and dispel magic; the blue 30'×30' for ice storm; the brown 40'×40' square for sleep; and the green 40'×50' rectangle for green dragon breath.
Top right, we have some circular templates—the pink 10' radius for, as you might guess, protection from evil 10' radius and invisibility 10' radius; the brown 15' radius for light and silence 15' radius; the orange 20' radius for good old-fashioned fire ball; and the large black 30' radius for torches and lanterns, continual light, a whole bunch of other miscellaneous spells, and my own house ruled version of turning undead. The yellow 60' line is, of course, for lightning bolts.
(Side note regarding light: making up these templates just now has reminded me how very easy it is to forget about light sources whenever a combat breaks out in the dungeon. But even continual light is only a 30' radius; which means that I've been seriously neglecting the degree to which monsters can use darkness to their advantage in a fight. Well; no more of that, I say!)
Next, cones: we have the 50' cone in green for chimera and hell hound breath, the 60' cone in black for catopblepas/nekrozon and nuckalavee breath, and also black dragon acid; the white 80' cone for white dragon ice, and the red 90' cone for red dragon fire. And finally, just beneath those, the long, narrow 100' blue line for the laser-like area of blue dragon lightning.
I can't wait to see the looks on my players' faces when I bust these out at the game table this upcoming Saturday. The party magic-user just made 5th level at the end of the last game session, after all, and he's been itching to sling some elemental attack spells!