I stumbled across Siamese Cat shortly after moving to Tempe many years ago and it soon became one of my favorite places to eat. It's a small restaurant run by a Thai family so you know the food is authentic. The service is excellent (very fast, very friendly), the prices are low, and the food is fantastic. I've tried nearly everything on the menu and have never been dissapointed. I especially love the spring rolls (Pau Pia), anything with curry (especially the Curry Noodles), and the home made coconut ice cream.
I've brought lots of friends and family members with my over the years and every single one has loved it, even those who had no previous experience with Thai food. I really can't recommend it enough.