Distributing gifts is an essential element of Christmas, and Secret Santa is a fun party game that brings this element to your Christmas party.
You'll have to notify all your guests in advance that you'll be organizing a Secret Santa at your party, and you'll probably have to meet up with them as well.
The basic idea behind Secret Santa is that each guest randomly draws another guest and then has to buy a gift for him. The other person usually never finds out who his Santa was... well, sometimes Santa's tend to come out after the gifts are given.
Because each person only buys one gift it's budget responsible and since they know who they're buying for, the gifts can still be personalized.