Making the webapp IE-friendly

I (like to) believe, the hackers community here are very Mac-centric. (If not, that's okay, don't flame me). I have been working on a RoR app on Mac, so most of my testing (80%) happens on Firefox, and about 20% on Safari. Today I happened to check the interface from IE - it truly sucked! Layout gone wild, JS issues, the bground color missing on most divs, turning the whole app to be a bland unusable site. How do you guys develop your app? How do you make X-platform compatibility test? I know its a very generic question, but would like to get your viewpoints anyways.