What are some good recipes using left over egg yolks from another recipe?

Mayo, hollandaise, zabaglione.

When I make lemon chiffon cake I use the leftover yolks to make lemon curd to put between the layers.
You can preserve lemon curd by canning it, but only if you use bottled lemon juice due to unpredictable acidity levels (http://nchfp.uga.edu/papers/2006/06ift-lemoncurd.html).

You can also freeze egg yolks if you mix a teaspoon of sugar into each yolk to keep the ice crystals from getting too big.

Madeleines use a lot of egg yolks. This time of year when I'm making my Amaretti agli Pignoli and Macarons au Chocolat, I also make Madeleines with all the yolks.

Here is a little lemon pudding that is delicious:
http://www.lorilongbotham.com/lusciouslemonrecipe.html

On Food52 there is the very popular Lazy Mary's Lemon Tart, I like any lemon tart. A press in shortbread crust will make it easy but you can use a sweet pie crust.
http://www.food52.com/recipes/1374_lazy_marys_lemon_tart

And I've made the Alice Medrich lemon squares posted here - truly the best:
http://www.food52.com/recipes/7853_the_tangiest_lemon_bars

Aioli!! There was an entire contest on Aioli, if you want more ideas: http://www.food52.com/contests/240_your_best_use_of_aioli The other ideas for using them in any custard are good. Or add them to a quiche or frittata. ;o)

Sabayon (an Italian or Southern French sweet custard) is made with egg yolks, sugar, and wine. I just added an Eggplant dip that requires one egg yolk too.

Mayonnaise, bearnaise or hollandaise sauces, fruit curds, pastry cream, ice cream...

French toast using yolks only in the custard is really good. Lemon curd is another great idea. Custards of other sorts.