Well, web2.0 has brought RIA with him (or vice-verse) and that means a lot of UX/ID/UI designers are fighting to express the behavior of those new type of web applications.
After exploring the different possibilities for expressing RIA behaviors I came to the conclusion that there are 2 great ways for doin it:
1. For client work – Guided Wireframe Narrative – it is just great! – found this great article about it.
2. If you develop your own application – just sketch and immediately prototype it in html (37s way…).
Try it.
