I use 1Password. It has versions for Mac, Windows and iPhone, with extensions for all the browsers I use. For the rest they offer an HTML version of your keychain that works almost like the native app, so it is very convenient.

The only thing that is missing is a sync service (they used to have one but they chickened out). The keychain file can be shared on DropBox or something, so that kind of solves that problem, but it is not ideal :)

I haven't tried many alternatives, but so far I am happy with this solution :)