Google Gears and Webkit Nightlies
I’ve been using Google Gears with Safari the past few weeks, primarily for WordPress 2.7’s “Turbo Mode” (useful on low-bandwidth or high-latency links), and secondarily for Gmail’s offline mode.
The currently released build (0.5.12.0) of Google Gears isn’t compatible with the latest Webkit nightlies, however. Good news, though: it’s a piece of cake to build it yourself.
Check out the source, and build it locally on your machine. I had no hiccups with OS X 10.5.6 and Xcode 3.1.2, everything Just Worked The First Time.