I'm only on two OSes (Windows, Linux) and predominantly two browsers (FF, Chrome), but Keepass (Works in Windows and Linux - pretty sure Mac too) + KeepassHttp plugin + PassIFox [1] + ChromeIPass [2] works for me.
I find it also nicer than LastPass as it combines with the native password saving of FF/Chrome rather than the highjacking and restyling of input boxes that look like user/passwords done by LastPass.
It's probably fine so long as both your 'client' and 'server' are on localhost, but it'll probably never be secure if the server is exposed to the internet.
I find it also nicer than LastPass as it combines with the native password saving of FF/Chrome rather than the highjacking and restyling of input boxes that look like user/passwords done by LastPass.