Do you have a digital product such as an ebook, video or audio product?  Maybe a software product of some type.  One of the biggest benefits of having digital products in my opinion is that you spend time at the beginning to develop your products, then you get paid forever.  It's kind of like having an "internet vending machine" for each product that you have.  That is, if its a download-able product.  The down side of downloadable products is that there are some pretty sharp crooks out there that will dedicate a lot of time to figuring out your download folder.  Experienced product hackers have lots of tricks up their sleeve which they can use to figure it out.  Fortunately, a vast majority of your visitors are pretty honest, but it only takes one.  Now maybe it wouldn't be so bad if they just kept it to themselves, but that's not normally the case.  These kind of people like to show off their talents by sharing your products with pirate forums, warez sites or even illegally selling your product somewhere like eBay!

So what's a product owner to do?  Well, you can give your download folder a really funky name with lots of numbers mixed in to slow down the hackers. You can hope that your download folder won't be shared with the world by some shady customer, or you can use a security product like Download Guard.  Download Guard was created by Sam Stephens and protects your download links like a fortress.  This is a full blown security suite written in PHP and has an easy to manage admin area to setup your download links.  I use Download Guard with both my Clickbank and Paypal products.  DownloadGuard handles it all.  DownloadGuard even handles your thank you email automatically, so that everything happens without the need for any intervention on your part.  You just setup your product, plug in the info to Download Guard, generate a payment link, and that's it.  When you make a sale, an email is automatically sent to your customer with the protected download link.  The customer clicks the link and is taken to a professional download page where they can click the download button to retrieve their product.  The link is dynamically generated and encrypted so the recipient never knows the true location of the download folder.  You can also add bonus products to your download page.  Also set the links to expire after a set number of hours.  I usually set mine for 48 hours.  If your customer misses their window for some reason, you can simply reset their link for them.  

I've been using Download Guard now since the very beginning.  I released my first product in 2005 and purchased Download Guard within a couple of weeks afterwards.  In the beginning, I had an issue with my web host and DLG.  I contacted Sam Stephens via his support forum and he went well above and beyond the call of duty to get me up and running.  It turned out to be my host, but he actually created a fix into his product just to address it.  He even tied into my FTP account to set everything up.  It was incredible and he set an example for me right from the start.  Over deliver on making your customer happy and you'll have them for life.  Download Guard was without a doubt the best investment I've ever made for protecting my digital assets. I don't just recommend things lightly.  This one gets my 100% endorsement. 
Check it out!