Usually, distros that are meant to be used for desktop come with Firefox pre-installed. Other popular browsers would include: Opera Google Chrome Seamonkey (using Mozilla, similar to Firefox) Galeon (isn't precisely a web browser, it's a part of Gnome, but it can be used to display web pages just the same, it's using Gecko, i.e. same as Firefox and Seamonkey) Konqueror (another Webkit browser, similar to Google Chrome)
All of the above are capable of running JavaScript and have NPAPI (well, I think so...) implementation. There are w3m and Lynx pagers / text-only browsers, they are of no particular interest to Flash because they only display HTML+CSS. They are good for reading stuff like technical documentation though, because they are much faster and more accessible. There might be few more, or probably some old not supported ones, but the browsers I listed are likely to be found on an average Linux installation of a web developer :)
