In my case fastmail.fm, zenfolio.com, and pinboard.in.
How is their web UI compared to Gmail, for example? Do you feel like you're missing out on any features, or do you access mail exclusively via a desktop client?
They have a really nice web GUI. I use it for access to my mail when I'm away from my laptop, where I use Mail.app. I haven't used gmail enough to do a direct comparison. The search is good, and you can set up very complicated rules to file mail into folders.
fastmail.fm. Their webmail is pants (all webmail save GMail is pants), but their IMAP service has been 100% bulletproof. Oh, and MobileMe for pics, but that's because I'm trapped in the MobileMe universe because the value of OTA contact syncing is just ... barely ... enough, and now most of my pics are there. Vendor lock in is easy; the first hit (when I was an employee) was free.
I pay for Fastmail too. But I hate myself for it. Gmail offers IMAP nowadays, no?