Thought it was time for an update....
I have been doing this 6 months. What I have noticed is that all sites are not created equal.

Each site has it's own rules on what is an acceptable image. So I just upload all that I have processed and what they take is what they take some sites will take them all, some only half of them.

One big thing that I am seeing is that is seems that the sites that are harder to get images in, by that I mean you are restricted by the number of uploads are also the sites that seem to have more traffic. I say this due to the increased sales that I have on the sites that I have fewer images in my portfolio. That being said these are my top three performers.

iStockPhoto,
Dreamstime &
Fotolia.

Update... I have been on ShutterStock for 9 days and have sold 81 images, this is the site to join. It did take me 3 trys to get approved but it is well worth it. I can't stress enough to get on ShutterStock.

There is also a big diffference in uploading to the different sites. Some like Lucky Oliver make it real easy others make it more difficult and by difficult I mean time consuming.

So if you are going with multiple stock sites you will have to learn the nuances of each site.

Most of all have fun with it. I look at it as every image I take just makes me a better photographer and why not get a paid for it along the way.

Look below for Reviews on each site I am on. The first ones are done for ShutterStock and iStockphoto.

All in all the more you upload on a consitant basis the more money you will make.

My top Money Makers
1 ShutterStock
2 iStock Photo
3 Dreamstime
4 Fotolia
5 BigStock
6 StockXpert
7 123RF
8 CanStock
9 Crestock
10 FeaturePics

ShutterStock

ShutterStock rapidly has become my best money maker. It shocked me a bit at
first with only making 25 cents per image I did not have much faith in it, but
by the second day images were selling extreamly fast compared to the other
micrstock sites. ShutterStock is mainly a subscription site where buyers pay a
monthly fee and can download x number of images per day. They have increased the
25 cents to 30 cents for all photographers who have made at least $500.

ShutterStock has no daily or weekly upload limits which some of the
other sites do. They have been approving my images within 48 hours but it is
usually with in 24 hours. They have FTP access for uploading but you need to
contact them to get it turned on for you. Otherwise you are stuck
with uploading 10 at a time via their website. Once uploaded you are only
two steps from waiting for approval. First it checks your keywords
then it confirms you selected a category to each image.

Reporting is good and is updated every hour, it's not reat time but close
enough. You can get a daily breakdown on all image sold and view the images
that have sold. You can also get an over all picture of your best selling
images, they list how many times each image has been sold.

Payments are sent once per month - by the 15th of the month - for earnings
from the previous month. The minimum payout is set in your account
options. The default minimum payout is $100 - but you may set it to $75 if
you would like.

Overall I have to say that this site is my biggest money maker and I
have only been on it for 9 days. It has passed the ohter eight sites I am
on some of which have been 2.5 months.

Upload Limts = None
Minimum Payout = $75
Payment Types = Check, Paypal & Moneybookers

iStockphoto

iStockphoto is a difficult site to get into and they have a high image
standard when it comes to approving photos. Probably the highest standard of all the sites I am on. They also have a high volumn of traffic, Currently the highest of all the sites, which converts into good sales. I have been averaging .57 cents per sale with this site. The royalty structure on iStockphoto is 20%. If you're eligible for exclusivity, you can make up to 40%.  You will need 500 downloads to qualify for exclusivity.

For newbies there is a 15 images per week upload limit which makes building you portfolio up difficult. It also takes about a week to get images approved. You also have to upload one image at a time via thier website which is no time saver.

I find the reporting a little different then what I like but everything is
there you will need. They also have a nice widget for your desktop that alerts
you of new sales.

Overall I am pleased with this site but wish I could build my portfolio a bit
quicker then a max of 15 images per week.

Upload Limts = 15 per week for new users
Minimum Payout = Balance of$100
Payment Types = Check, Paypal & Moneybookers

Going out of business

Lucky Oliver

Lucky Oliver has a carnival type of atmosphere to it's site. It is relativly new to the microstock game so traffic seem much lower then some of the other sites that I am on. The staff  aka The Bouncer Team have been the most helpfull with their comments on my submissions. Not only telling you what is wrong with an image but encouraging you with compliments on good photos.

Lucky Oliver also has the easiest submissions of all the other sites. You can
FTP upload, then just submit them. No categories or anything else one click of
the mouse. Probably why I don't mind uploading to them even though they are a
poor performer at the moment, but I'm sure in time the site's traffic will pick
up and will become a good money maker.

There is no limit to uploads and thus far they are one of the easiest to get
your photos in, a lot less critical but I bet that changes once they become
more established in the microstock community.

Upload Limts = No Limit
Minimum Payout = Balance of$100 (As of 03/08 the new limit will be $25)
Payment Types = Check, Paypal & Moneybookers

Dreamstime

Dreamstime is another good performer when it comes to sales. Users with an
approval percentage higher than 60% can upload the maximum amount of images (25) and can use the FTP feature. This applies for the first 100
submissions if you're a new photographer. Dreamstime also is very strict on the
images they approve so don't be discoraged if you get a lot of rejections. You can upload more than 25 images a day but can only submit 25 of them and submit the rest over the next couple of days depending on how many images you
uploaded

Uploading is made much easier via FTP their onsite upload feature is for one image at a time so I would suggest taking advatage of  a FTP program. Be sure to go to your Unfinished files area to complete the submission of your
images you will need to add categories at this time. One feature that is nice is that if you have many images covering the same subject it allows you to copy your keywords and or categories form another recent submitted image.

The reporting on Dreamstime is pretty good, it allow you to sort by any of the columns to get the results you are looking for.

Overall I would reccommend Dreamstime to any photographer for it's ease of
use and reporting. Of course sales and traffic are good too.

Upload Limts = 25 per day
Minimum Payout = $100
Payment Types = Check and Paypal