I'm a fan of MooTools, mostly because it's pretty lightweight, easy to use, and if you're only looking for basic functionality or effects you don't have to get bogged down with the other stuff. Although Prototype and script.aculo.us are the basis for a lot of Ajax and JavaScript work, so you might end up using them regardless (Rails has them baked in, for example).
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Nice tutorial.
I'm a fan of MooTools, mostly because it's pretty lightweight, easy to use, and if you're only looking for basic functionality or effects you don't have to get bogged down with the other stuff. Although Prototype and script.aculo.us are the basis for a lot of Ajax and JavaScript work, so you might end up using them regardless (Rails has them baked in, for example).