Keychain. It's part of OS X & will store passwords, secure notes etc. & is likely to do everything that most people need.
Ideally (and this is not default OS X behaviour, but is what I do), make your keychain-unlock password different from your login password. You do need to remember two passwords, but it prevents anything (such as mail) from retrieving a password that is stored in keychain without you explicitly permitting it each time.
For enhanced security, don't use an admin account for your day to day operations (i.e. create a 'user' account & routinely use that one. Reserve the admin account for admin-things). Store applications which you install yourself in a local-to-your-account applications folder (which you can therefore write to as a non-admin user). That way, nothing you do can touch the system.