This is the latest Review/Giveaway we were involved in http://www.joanandriahs.com/delta-moon-soaps-hearts-for-your-valentine-review-giveaway/
The way it usually works is, you give the reviewing site one of your products to review and if they happen to enjoy your sample, they will do a review and host a giveaway for it. You will then provide the giveaway once the winner has been chosen. Here's the reason to get involved in these:
You get lots of exposure from the blog, which will promote the giveaway as well. If the blog has a lot of Twitter Follwers, Facebook Followers and regular followers of the blog, they will all learn about your poducts. It basically ads to to a ton of exposure for the cost of two soaps (for me). I have done a couple of these now and have found them to be very worthwhile, I don't know where else you can get that kind of exposure for so little expense. Give it a shot!