I would like to throw my hat in the ring with MooTools. Seriously, anything that can make cross browser Javascript easier gets my vote any day. Of course like anything else there are draw backs, including some effects deficiencies in Internet Explorer. Although how can any Web Developer take IE seriously these days?

I know I cannot do it since I spend 80% of my debug time trying to work out some CSS or Javascript glitch that Internet Explorer does not address properly. I will admit that IE8 does address some of these issues with the new commitment to standards, however in my opinion it is a little late considering that the latest version of Internet Explorer will break the majority of websites that employ IE hacks who are trying to make everything look good.

Anyway, MooTools is awesome. If you have not chosen a javascript framework yet check it out, it is worth at least a look.