A possibility for opposing pending patent applications in the US is presented by Dennis Crouch in his blog Patently-O under the title "poor-man's opposition proceedings ".

The US law does not allow opposing a patent application (in contrary e.g. to israeli procedures which are partly based on receiving third party's oppositions), however they do commit applicants to submit to the patent office any relevant subject matter.

So, the procedure comprises sending such subject matter to the applicant, thus making him obligated to disclose it to the patent office!

I like this idea.