Here's a simple spreadsheet that lets you quickly make a list of guests, enter the cost per guest - by type (adults, kids) - and then get a total cost for the party. Seems easy enough to do on paper - but this saves tons of time when you start getting RSVPs or adding/removing guests. Also helpful to track who's bringing what to a less formal party - although future versions of this will incorporate forms for collection of that info along with RSVPs (look for that change soon!).
Go a party coming up? Get your own copy
Here's a preview of the Party Planner Spreadsheet: