Distribution is  very important in  stock photography and a  quite  time  consuming task. when uploading to multiple agencies.  
Isyndica lead the way but unfortunately closed down in 2010. Since then   some new tools are available either online or via a desktop program.
Microstocksubmitter,  developed by microstock contributor Yermolayev Konstantin is a  standalone application capable of  uploading  pictures to many stock agencies. It is not an "upload once "product, however the  big advantage  is that, you can  (for 7 agencies so far) select  categories  and  your attach model releases from the application itself  which  is a real time saver.  Pop up window sale reports are also available and are working  for DT,SS,FT,CSP, 123RF,IS, Veer at the  moment.
Automatic  submission  (model release attached and submitted)
Shutterstock,Fotolia,Dreamstime,DepositPhoto,Veer, Canstockphoto,Bigstockphoto,123RF,Istock,Graphic Leftovers.
Final submission and upload of  model releases necessary: Stockfresh,Scanstockphoto,Yaymicro,Zoonar,
(MR not needed), mostphotos (MR not needed).
 
With  the free version of Microstocksubmitter, you can distribute up to 50  files per channel per month. If needed, you can upgrade your  account to submit up to 333 files  for  $7  a  month or  choose  the    unlimited uploads option for $15 a month. A version for unlimited uploads  not limited in time can be purchased for $250.
Pros: free account available, automatic final submission (categories,  model release) for the main 4 stock agencies. Frequent updates.
Cons: the same has to be uploading to each channel
If you are interested by  
Microstocksubmitter I will appreciate if you sign up under my email ldambies@gmail.com. Thanks:)
 
 PicWorkflow (referral link included)  was developped by  Robert  Davies, also behind the development of 
Picniche. Taking the philosophy of Isyndica PicWorkflow is a powerful platform to upload once and distribute to multiple agencies. Statistics and  promoting tools as well as retouching are also avalaible.
 
For  cost calculation, files are segmented by 22MB blocks so most of  pictures will cost 1 credit (1 US cent) to distribute to one channel and  a 35MB photo for example will cost 2 credits to distribute. A 500MB  movie file would cost 25 credits (25 US cents) to distribute per  channel.
Once  uploaded using an ftp client like Filezilla,  files are stored seven  days and subsequently deleted.It is however possible to extend their  storage life by paying an additional fee. I am using picWorkflow to syndicate videos to Pond5, Clipcanvas, Shutterstock  and Revostock and as a former Isyndica user, I am very pleased with the service so far.
 
Pros:  Upload once, multichannel distribution. Footage distribution. Storage option available.
Cons: Login necessary for final submission of pictures. no IPTC reading for footage. Interface not so intuitive.
LighBurner, like picWorkflow, is another  ''upload once'' distribution tool developed by David Mail already behind  Prostockmaster and Pixamba. It is hard to believe but Lightburner is free so far to distribute pictures to 13 photo agencies.  Like picWorfkflow, you have to  send your files by ftp then login into the online platform to distribute your work. Lightburner looks  promising as Analytics and  promotion tools will be included soon as well as a  full integration  with Prostockmaster. I expect also the platform to be capable to distribute footage in the near future and it will be interesting  then to compare  fees with picWorkflow.
 
Pros: Free. Upload once, multichannel distribution.No upload limits. Automatic final submission fo some agencies. 
Cons:  No footage distribution. Pictures get erased from the server after 3 days with no option available for permanent storage. Bugs still exist (duplicates).
- What are the challenges for distributors?
Challenges are both technical and financial. for distributors. We saw in the past with Isyndica that a great distribution platform alone is not enough, the venture has to be profitable as well. Due to the small amount of contributors willing to pay for this service   and the because of the small margins, it has either has to be a one man operation or one of the tool of an existing company.
Distributors can also provide promotion, keywording, statistics, retouching  tools which can add value to their product and increase their margin.
Regarding the technical part, contributors with paid accounts expect a reliable good technical support and frequent updates from the distributor and it is not easy task as  agencies do not not really support them.
Due to the storage place, distribution  is expected  to be a challenge as well especially with heavy contributors who have 1000's of clips ready to upload.