If you have an account with a company whose servers have been hacked, it’s nerve-wracking to wonder whether or not your private data has been leaked onto the Internet. Thankfully, a new Web service seeks to aggregate all the leaked account data on the Internet and make it easy for you to check and see if you’re on the list.

PwnedList (pwnedlist.com) is the brainchild of Alen Puzic, a professional security intelligence researcher partial to a bit of "white-hat" (good-guy) hacker work. PwnedList was born in July 2011 as a public service to help privacy-minded people verify the security of their online accounts.

“Our goal was to design a simple-to-use online portal where an average user could check to see if his or her account credentials were leaked,” said Puzic in an interview with PCWorld. Within a week, Puzic and his team (including security researchers Stephen Thomas and Jasiel Spelman) had gathered more than a million hacked accounts from websites like The Pirate Bay and PasteBin, social networks like Twitter, and even hacker forums and chatrooms. At the time of the interview, PwnedList had been operating for almost six months, with its database approaching 10 million entries.

But don’t worry: Even though the folks at PwnedList are constantly seeking out compromised usernames, email addresses, and passwords, they don’t store all that information in the PwnedList database. Instead, they take all the compromised account data they find (or that anonymous users submit to them) and use an algorithm to create a unique string of alphanumeric characters for every username and email address. They then save the strings in the PwnedList database before deleting the actual login information. This procedure means that no hacker can crack the PwnedList database and gain access to a single list of the hundreds of thousands of compromised accounts that the PwnedList team is aggregating.

So every time you type a username or email address into the PwnedList search engine, the server runs your request through the same algorithm used to hash the compromised accounts, compares the string generated against the strings in the database, and alerts you if there’s a match. For extra security, you can even avoid typing your email or username into the PwnedList website by hashing it yourself and copying the string. PwnedList uses a 512-bit Secure Hash Algorithm (SHA) hash, so you can just use an online hash generator to convert your favorite email or username into a string of gibberish.

Of course, since the PwnedList database is just a giant list of alphanumeric strings without relevant data like passwords or domain names, the service can tell you only whether or not a particular name or email is on the list; at the time of our interview, PwnedList offered no way for you to know exactly how your email was compromised or which site was hacked. That will probably change with the next version, though.

“We're working hard to make more metadata available on our site...including the name of the site/company that hosts the account, the number of accounts contained in the leak, the date we found the leak, and (if possible) the name of the hacker/group that we believe published the data,” says Puzic.

But ultimately that extra data, while helpful, doesn’t really matter; what matters is that sites like PwnedList help you take an active role in verifying whether your private data has been compromised. If you’re unlucky enough to find your favorite username or email address on the list, don’t panic! Chances are your data hasn’t been compromised yet, but to be safe, you should assume that you’re the victim of a data breach and take a few common-sense steps to recover from it. Update all your accounts with better passwords, put a fraud alert on your credit report, and monitor your financial statements for a few months for signs of tampering. For more tips, check out our guide to recovering from a data breach, and keep an eye on PwnedList as it continues to roll out more privacy protection services.