The rise of commonly available JavaScript frameworks means you don’t have to worry about all these annoying differences in JavaScript implementations between different browers. They are so comfortable to use because they allow you to use lots of free available plugins (esp. jQuery) and forces you to access higher level JavaScript APIs that are guaranteed to work across multiple browsers.
The most popular JavaScript frameworks are (see also the overviews here and here) :
1. jQuery and jQuery UI
2. Prototype and Script.aculo.us
3. Dojo
4. Moo Tools
Personally I like both Prototype and jQuery, but jQuery has a lot more useful plugins to offer. jQuery is perhaps the most popular one currently, check out the slides from Yehuda Katz from the Rails Conference here
4 Lines of Code 
I haven’t checked out any JS frameworks yet but I’m considering checking out jQuery as I’m starting to do more complex JS animations and I think it would seriously cut down dev time for such tasks.
The overhead of the framework files is always something that kind of deters me, but since Google is hosting jQuery now I may look at it more seriously because that’s a copy that could be cached on quite a number of machines.