Even though I prefer accessing my mail and notes through IMAP, and my calendar, contacts and files through DAV using native apps on my computer and phone, I love using Fastmail's web user interface. It's fast, it's responsive (UI is simplified on mobile but it's still the same design; not an alternative layout) and it looks really good.

Site is down because of bandwidth limits. This is also a Fastmail feature: you can serve a directory uploaded to your file storage through HTTP. Even though there are bandwidth limits, it works as an alternative to GitHub Pages or to just make available some files you want to share with multiple people. However, it seems it doesn't survive a Hacker News hug of death.