I use Lastpass, which has a client for most platforms.
They could be compromised, but they claim all data is encrypted locally before being sent to their servers. I have not verified that claim.
A password manager does not encrypt or sign mails.
Sorry, I wasn't clear. I use last pass to transfer my private key to the PGP app on my iPad (and elsewhere) (as opposed going through Dropbox or whatever).
Relies upon trusting last pass and trusting the iPad of course, both of which are questionable.
First, I'm not remotely interested in running my own mail infrastructur anymore. Been there, done that. Today it's much too hard to get mails accepted by others.
But more important: iPads don't have an USB connector, my mobile phone doesn't have one. Friends have Macs, in other places there might be other crippled devices.
The web is a universal building block. USB sticks are not.