Tithe of your increase.
When I get unexpected income, I do something creative with the tithe -- give to friends in missions, a shelter, or someone that I know needs temporary support ... you might even think of what the deceased people would have liked. I don't think a tithe has to go to a religious organization, because the original tithe was distributed to community programs as well as priests. (Think soup kitchens, volunteer ambulance...) If you are worried about where God's borders are, ask Him.
Consider that you have to live with yourself, and start from there. Pray about where you might give, and an idea will probably come clearly and immediately (or maybe it hasn't).
A donation is a tax write-off anyway, in a year that could put you in another bracket, so it's not like you're losing the entire ten percent.
The reason to give is not to fulfill an obligation. It is a wonderful thing to have enough that you can help others out with your resources.