Starbucks is well-represented in greater Phoenix. If you have a "Starbucks gift card" and register it online, you get 2 hours free wiFi each day*-- so long as you use the card or add money to it at least once/month.
The Starbucks in Kierland Commons is geared to the transient coffee-house-as-office customer, with two desks with outlets. Each desk easily seats four people (and laptops).
* I've used the wiFi for more than two hours at a sitting and AT&T did not kick me off the network.