Google's Matt Cutts tweeted that Google has a form webmasters can fill out if they think they've been wrongly hit by Google's Penguin update (also known as the Webspam Update).
The link takes you to a page that looks like this:
It's interesting that Google is giving users a form this time. With the Panda complaints, Google was trying to have them all directed to a particular thread in its Webmaster forums, though the thread eventually got broken up.
On the form, you'll notice that Google tells you to go to a different page to submit a spam report, and to use the word "Penguin".
@mattcutts Totally forgot to add the word penguin in the spam report, still going to get any attention?
@DanielDeceuster might want to submit again and include penguin just to be safe.
@mattcutts Thank you! We were seeing some major effects and wanted a good way to report some poor quality sites coming up in search.
@ScottJConlon we're definitely interested and we'll be reading the feedback from the forms.
Keep in mind when doing your reporting on either side of the equation, that the Penguin update is specifically aimed at sites that are violating Google's quality guidelines. Here, you can read about what those are exactly.