Russel Wilson of Dexo Design has done an excellent review of available UI Prototyping tools and provided tables comparing attributes including how fun they are to use, time to ramp up, and support for interactivity.
His final assessment is below:
1) Low fidelity mockups for idea exploration and communication: Balsamiq
2) Interactive prototypes to test and communicate interactions: Adobe Flex (Flash Catalyst/Thermo may change my mind)
3) High fidelity drawing tool: Adobe Illustrator or OmniGraffle