To achieve the same cheaply and without lockin, simply have your own domain with a catch-all email forwarding to your real email address. Then always give out your email address for a specific site as <site>@yourdomain.com.
I have my own "yourdomain.com". I pay to keep it registered and keep a server running also. Most people don't have this; their mail domain is "gmail.com" or whatever. Sure, a lot of problems could be solved if everyone just had their own domain!
Speaking of "gmail.com"; I'm surprised Google doesn't just make this a feature of gmail. It would be fairly trivial for them to implement for the benefit of all gmail users.
Occasionally you'll run into a form with broken email validation that won't let you use a + character, but I've been doing this for years and it works the vast majority of the time.
This type of thing can work, but only for a small-time service provider whose plaintext encoding scheme is not widely known. (Security thorugh obscurity.) Even the hard-core spammers won't sift through millions of e-mail addresses to crack some plain text scheme that is used by two or three of them.
Also, you need the option to permanently destroy one of these, so that you never see mail from it again. No filtering bullshit. Google should control the exact set of anonymized addressees attached to your account. When you destroy any one of them, any further attempt to send to it should result in a non-delivery notice (SMTP bounce).
Suppose I have two users in my domain: bob@mydomain, alice@mydomain.
How can alice just make up a new @mydomain address which goes to alice@mydomain? Okay, that part is simple: we can have an entire space of these generated by a rule, like gmail's addr+whatever@gmail.com.
But then how does alice invalidate such an address that is misused?
I want it so that any address that is not valid generates an SMTP bounce; I don't want an infinite space of aliases that map to an address to all be considered valid, but a specific set, controlled by the user. When an element is removed from that set, then further attempts to send to it generate SMTP bounces.
Furthermore, I want it to be completely anonymized, just like Throttle are doing, as in:
<random-chars>@mydomain -> alice@mydomain
EDIT:
To solve one of these problems, what we can do is assign to each user some random identifier of fixed length, from which further addresses can be generated. For instance alice@mydomain also gets "xZa3f@mydomain" when the account is created. To this local part, arbitrary characters can be appended: "xZa3f4abPspamming.dickheads.com@mydomain" such that this still routes to alice@mydomain. Doesn't handle the SMTP-level invalidation requirement though.
As people have pointed out, it now seems possible to use the + functionality of gmail which I was not aware of, but the above setup avoids the issue of forms not accepting + in a valid email address.
This only works with a catch-all email forwarding as I mentioned, so if you want bounce on non-valid addresses it will not work, but like I said it has served me very well.
The problem with this solution is that many email harvesting widgets incorrectly see +xyz as invalid, even though it satisfies the rfc just fine.