The Mac's Keychain Access program (Utilities folder) is pretty good for this. Most programs I use directly support it, e.g. Mail passwords, and web site passwords in OmniWeb. You can also add your own passwords or secure notes without having a program "support" the keychain.

Sync of keychains is possible, but only if you pay for MobileMe (nee iTools/.Mac).

Unfortunately, Firefox uses its own password manager on the Mac instead of a keychain.