1) Houseparty - Now this is my personal favorite, which I have been a member of for the longest and it is very easy to start. You fill out a few surveys and tell them about yourself then you find out what parties are now receiving applications. You apply and do minor actions to increase your chances (let's say by sharing on Facebook or twitter and also reading the announcements and starting conversations). If you are chosen to be a host, they usually give you 30 minutes to fill out a quicks 2 minute survey and inviting at least 12 people via email mainly. You then get your package in the mail withing 2 weeks and have your party, take pictures and have fun. The most wonderful thing about houseparty is that the host gets the most amazing items to keep and your guest are just as satisfied. I have tried items from Fisher Price, Hot Wheels, Glade and Huggies before they go on the market.
2) BzzAgent - This W.O.M. Is also just as good as houseparty. Basically you get do surveys so that they are able to get to know you and what you like and would be willing to try and buzz about in reviews and face to face. There are new campaigns almost every week that I have noticed and based on your answers in the surveys, they send you an email inviting you to try the campaign which you can decline if you don't like it. You are more likely to get chosen quicker than houseparty and it takes less actions to get an invite. Keep your buzz score (bzzscore) high and the more likely you will be invited and for better ones. I am currently trying Courvoisier Gold, Courvoisier Rose and C by Courvoisier and there are lots more to come.
3) Smiley360 - this is a fairly new W.O.M. which is not as great as the first two mentioned but do have some really good brands to try. You do missions
http://smiley360.com/1375160.cfm to gain smiles which would help you get better missions and score better and bigger brands. The only downfall is their missions aren't as frequent but worth it.
4) Moms Meet - this is a W.O.M. is mainly for moms or people who want to learn about a greener living. You could read about this particular website in my March 1st blog and find out more info
5) Swaggable - this website is also very new but pretty interesting. You don't need to do much to get chosen to try an item. Basically, as the norm you full out the info about yourself and what you like and dislike. Then there will be a list of products that you may (want) to try and you just click them. Based on my experience you get invited to try mainly the newer products based on your want list. There aren't many products but are very new products and very unique (the average person would definitely not pick it up during their grocery run).
I hope you like this list and take the time to join and get new items.