Chrome/Firefox: When you sign up for a new web service, you're usually asked to hand over personal information, even if it's only an email address. MaskMe is a browser extension that lets you hide your information.
MaskMe essentially creates a dummy account that forwards your details. For your email address, for instance, it creates a fake address so you can register at a web site, and then forwards emails to your real one. This way, no service has all of your data in one place.
The same basic process also works for credit card numbers, but you'll need to pay a $US5 a month fee for that option (and it might not work for every card). If you sign up for a lot of different services, the email masking alone is worth checking out.