For those in the market for a family plan today, T-Mobile is waiving activation fees on 2 or more lines. Unfortunately this offer is for today only.
AT&T is also waiving activation fees through the rest of May. That's a savings of $36 on one line and up to $150 on a five line family plan. (As of now (5/10), this includes the iPhone. However, we're not sure if AT&T is including the iPhone in the program all the way through May.
To compare all cell phone plans, and find the one that will cost you the least each month based on how you use your phone, visit our CellCalc page.