Zipster is dead & gone, see other answers below.
Zipster does exactly that.
blurb straight from their site
What is Zipster?
Ever been envious of the way Windows users can just
browse into Zip files, copy files in and out of them with ease and all
without running a Zip utility? With Zipster, your Zip files are just
like another folder in Finder. Plus, unlike on Windows, you can open
files and save into them from any application.
- Fast. Writes and compression are done in the background so you can get on with your work. And we’ve done a bit of work to make reading
- Friendly. No more pesky .DS_Store files — and your friends on other platforms won’t be complaining any more.
- Useful. In addition to plain Zip files, Zipster works with JAR, WAR, ipa and apk files.
It does have slight limitations when it comes to trying to put things away; it reacts a bit like a mounted .dmg except it's right where the file was, not on the desktop, but it's not a bad attempt at transforming .zips into folders.