A colleague just told me about the recently released Google Gears API. It enables offline development of web applications as it has got a inbuilt Localserver and an inbuilt SQLlite database. You can also write some code to synchronize your applications to work both offline and online. It is basically a browser extension and needs to be installed just once (I just installed).  Its currently in beta state (developer only release)  and is open source as well. I think this is something really cool.  I am going to try it some over the next few days and try to implement it where possible in my current projects. More details here.