I have heard about "Copyleft" and was wondering if there is an equivalent for works which fall under Patent law rather than Copyright law.
Specifically, is there a way I ensure that someone cannot patent an idea without filing for a patent myself?
"Copyleft" is a specific application of copyright law (restricting the license to use copyrighted material by requiring that those obtaining it must also redistribute it for free / under specified terms).
Any work under "copyleft" is protected by copyright law (thanks to The Berne Convention copyright is "automatic" in most of the world), and those copyright protections are what allow the author to dictate the terms under which the copyrighted work is used.
As far as I'm aware there is no Berne Convention equivalent in patent law: in order for you to have patent protection and the ability to dictate how your invention is used (by licensing the patent for it to impose Copyleft-style requirements on those using it) you must file for (and receive) a patent.
Note that the sort of "keeping information free" protection the Copyleft folks would seek is inherent in US Patent Law and most other patent systems -- it is known as Prior Art.
In its most simplistic definition (cribbed from Wikipedia):
Prior art …, in most systems of patent law, constitutes all information that has been made available to the public in any form before a given date that might be relevant to a patent's claims of originality. If an invention has been described in the prior art, a patent on that invention is not valid.
So if you have designed and publicly released something patentable (and can document the date of such a release to the satisfaction of the patent authorities) you can seek to invalidate any subsequent patent claims on the grounds that prior art exists.
There is not a direct analog to "copyleft" (informal but widely recognized term) in the world of patents.
As mentioned by the two previous answers, if your sole concern is ensuring that no one else can patent your invention, then public disclosure of the invention is probably your best course of action. However, you would want to make that disclosure as widespread as you are capable of doing so that patent examiners are readily able to find your invention to show it as prior art against anyone that attempts to file for a patent on your invention. Simply posting it to your blog is probably not going to cut it.
You ideally would want to get it published in some sort of widespread journal with a large distribution, and publicize your invention like crazy so that there are multiple venues through which an examiner would run into your invention. Of course, that course of action is not always feasible.
"Public disclosure" of an invention, meeting the basic requirements of a patentable idea, notably "enabling" it, so that it teaches someone "of ordinary skill in the art" how to duplicate the invention, will generally serve as prior art, barring others from subsequently patenting an invention.
In fact (as faculty at research institutions are often warned) such "public disclosure is interpreted by courts as an intention to donate the invention to public domain".
For many years, IBM published a Technical Disclosure Bulletin, that was basically a compilation of the invention disclosures that IBM inventors had submitted to IBM which IBM had chosen not to pursue patents on. As the Wikipedia article notes, this was basically a defensive move to make sure that no one else would be able to patent these ideas and use them against IBM. There was a lot of good stuff in there, particularly in the software area before software patents became common in the US. As an incentive to submit stuff, they gave employees credit for a third of a patent for each disclosure that was published in the Bulletin (1 point for a disclosure, 3 points for an actual patent, and you got special awards when you hit 12 points or higher, if I remember the system right).
So that's a historical example of a "PatentLeft" on a fairly big scale.