How To File a Reinclusion Request
Google, Yahoo, and Bing Reinclusion - Getting off the Search Engine Blacklist
Search engine reinclusion requests are easy to file, but require a lot of up front work to be successful. This is because you need to be sure that there aren't any factors on your site or in links to your site that would prevent an individual (who may personally read the request) from denying your request. If you've been blacklisted in Google, you normally will know it because your site is no longer cached and doesn't show up in the normal search engine results. You can sign up for Google Webmaster Tools to see if this is the case, since they aren't afraid to tell you that you are out of their index. They may not tell you why, but if you're a garden variety spammer you probably already know. If you aren't a black-hat type, you may have been delisted for running an affiliate site with no original (or "value added" content), for buying paid links from multiple other websites, for having a very spammy site with text that matches the background color of the site, or having elements like cloaking and doorway pages, even when they were meant for legitimate purposes.
Google Reinclusion Request info from Matt Cutts.
Reinclusion info from Google Help.
Find out more in the Yahoo Search Blog or fill out a request.
Bing Blog has general reinclusion topics plus Q&A. Or fill out their form.
Notes and Special Information
Special note: There are no guarantees of reinclusion. Sometime your site may be banned for bad behavior, even if it isn't your fault. You may also have multiple strikes against you.