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
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