Crack the egg into a cup or bowl, whisk it, and measure out half of the contents.
If you use eggs frequently, you could probably save the other half for a day or two – otherwise, it's like 8 cents out of your pocket.
cooking.stackexchange.com/users/219/ceejayoz
Freeze the egg and carefully saw in half, end to end.
Thaw and you're good to go :)
cooking.stackexchange.com/users/624/Adam Shiemke
Measure half by weight.
Simply crack an egg into a bowl on a zeroed scale, note the weight.
Beat the egg with a whisk or fork until combined.
Rezero your scale with a new empty dish and pour half the egg by weight into it.
cooking.stackexchange.com/users/60/hobodave
Depending on the recipe and number of eggs total, you can separate the white from the yolk.
This doesn't work well if you're dealing with just one egg total (throws the fat content too far off), but I've done this when going from 3 -> 1 1/2 with good success in baking recipes.
I've also used it to fine-tune the amount of fat in the recipe if I didn't like the original consistency.
cooking.stackexchange.com/users/161/acrosman
Depending on what you're making, there may be a good egg substitute other than that powdered stuff.
e.g. if you are making some kind of baked good, I've had good results from using bananas or apple sauce.
(bananas sometimes add a banana-y flavour though...
which is often a welcome addition :-))
There are a wide variety of egg substitutes out there, and each lend themselves to different kinds of recipes...
Perhaps one of these could solve your problem.
cooking.stackexchange.com/users/725/TJ Ellis
How do you halve a recipe that calls for 1 egg?  Clarification: I do not want to have to make the full recipe just to use half and I don't have powdered egg substitute.
How do you halve a recipe that calls for 1 egg?
cooking.stackexchange.com
<substitutions><eggs>
cooking.stackexchange.com/questions/1610
cooking.stackexchange.com/users/43/Dinah
Whisk the egg and measure out half, or see whether the recipe allows you to separate the egg or use a substitute. You can either whisk the egg up and pour out half, separate the yolk and just use that, or use an egg substitute.
Whisk the egg and pour out half? You could also freeze it and cut in half. Try separating the white from the yolk and just using one. It might be easier just to use an egg substitute.