Whether you have your blog or your website you can make money writing reviews. Of course you have to be part of Affiliate Network first. Once you are and trying to promote the product, the reviews are a great tool to help you.
Let's say you promoting "antispyware software". There is millions of people out there having problem with spyware and as much antispyware products. You want to convince them to buy yours.
Every normal person will read reviews or feedback for the product before they buy. So you have a chance that they will stumble upon your review with your affiliate link in it and you can make a sale.
The key of writing a good review is to balance it properly. There is no perfect product out there and everyone knows it. So you have to say honestly in your review what is good about this software and what is wrong with it. We want a reader to realize that it will help him to clean or protect his computer, that it's better in picking those little bugs then other programs available, that updates are daily etc
Then you have to move to the cons to make your review look honest.
What can be the cons?
Depends on the product. On this example we can say that interface could be more user friendly, customer support takes long to respond to emails, scan takes a long time or it doesn't detect specific trojan. You have to research the product you trying to sell before you write the review. But if you write the good, honest review you have 90% chance that they will click this link and make you money. That is unless you selling something what hardly have any pros.