Geo Places
This simple and well-designed search tool uses your current location to find things around you, such as restaurants, taxis, and banks. Use it as a simple and fast-loading alternative to Yelp or Google when you're on the go.

Hype Machine
Flash is so antiquated, so why are so many music streaming sites still stuck in the stone age? Not Hype Machine. The site uses HTML5 to aggregate all the hippest tunes from the most popular indie music blogs and lets you add it as a favorite and play it back. The site even has an iPhone formatted layout, and provides iTunes Store links.

Meebo
While there is an iOS app for this one, Meebo's HTML5 site is too handy when you're on someone else's device but are in need of a chat client. While there are better apps in the app store for AIM and GTalk, Meebo supports every chat protocol on the planet, so you never know when you'll need it.

Checklist
This simple checklist site allows you to make lists and email them. You can even use it when you're offline.

Google
Even though there are a variety of Google apps in the App Store, it's hard not to declare Google the mother of all HTML5 sites. It's hands down the best. And despite its representation in the App Store, you can do so much more on the site itself, and it's all easily accessible from a single page. Do yourself a favor and browse all that Google has to offer right from your iPhone!