I'm pretty sure MBNA, Discover and now Citibank license the same technology from an Irish firm.
The main features are:
- One-time use vs recurring payments option
- Up to 1 year Expiration date option
- Transaction $ amount limit
Someone mentioned Discover ShopSafe doesn't actually support the first option (one-time use), but for the time being, let's say all three banks do the same thing. In each case, it's a convenient Flash applet accessible in any web browser with the plugin. Discover also licenses a client-side native e-wallet app from Brodia, but I avoid anything that collects all your merchant passwords and personal info in one local database. That native app only runs on Winblows anyhow. I haven't used MBNA ShopSafe or Discover DeskShop recently (only Citibank VAN) so I may be a little off on exact details (but AFAIK all three use the same technology).
The point I'm trying to make though is that unless things have changed, these three banks offer more features than AMEX Private Payments. With AMEX PP, you get a one-time use CC that expires in 1 month and no more, unless they have upgraded things in recent months.