This is far from an exhaustive list -- it's always going to be a work in progress -- so if you know of addons I've missed that are ready to go for 5.0.4, pop a link in the comments or email me and I will add them to this list. There may well be many addons that are prepared for Mists' full launch in September but are not set up for 5.0.4 -- they're not what we're after here! If that's what you're looking for, though, there's a huge list of addons ready for Mists itself at Curse, and another at WoWInterface.
Please note that in the majority of cases, the addon page's live download will be updated at 5.0.4's launch, not several days in advance. Use your brain when downloading addons via these links; many of the authors who've contacted me have told me that their addons are ready to go but that they will not be uploading a release version to their live pages until patch day. This list of addons here represents those I have confirmed either through my own testing or directly from their authors that are ready for 5.0.4, and the links provided will be the links to download the addons on patch day or shortly before.
If there are addons you use that don't appear on this list, let me know and I will try to look into whether they're ready to go yet or if there is no news. I will do my level best to keep this list up to date and complete as we approach the launch of 5.0.4.
[I'd like to say a huge, huge thank-you to Bluspacecow for all their help with this!]