Online Work / Task

 There are different definitions for crowdsourcing and therefore different approches from different platforms.

Simple Tasks
Platforms like mechanical turk. Many workers do simple tasks for a little amount of money. Often tasks get done more than once and the result most of the people gave, is taken as valid.
http://mturk.com
http://clickworker.com/
http://microtask.com/ Mainly Digitizing
http://samasource.org/ non-profit, content moderation, digitization, data entry, Data Mining and Enrichment, Business Listing Verification
http://cloudcrowd.com/ "CloudCrowd provides Turk functionality and adds workflow functionality and innate quality control"
http://www.mobileworks.com/ quality through exclusivity

Metaplatforms
Because mechanical turk can be difficult to use, other platforms help people to structure their tasks and get better quality. They use mechanical turk as a ground layer and built upon it.
http://crowdflower.com/ uses mturk and samasource
http://scalableworkforce.com/ uses mturk
http://www.crowdguru.de uses clickworker
http://smartsheet.com/ collaboration tool with crowdsourcing provided by mturk and livework
http://www.serv.io/ media services, powered by CloudCrowd
http://houdiniapi.com/ “provides a bridge for integrating crowdsourced labor directly into applications using an API.”
http://cloudfactory.com/ uses their own trained workforce (Cloud Workers)

p2p
One person does the task. But the task can be split between many or done more than once.
http://www.optask.com/ Web Research, Marketing & Lead Generation, Procurement, Powerpoint, Excel and Word, SEO and Blog Writing
http://mediapiston.com/ content creation
http://newspepper.com video, photography and blogging services
http://textbroker.com/ text
http://livework.com/ Callcenter, Support (not exactly simple tasks)
http://becomeaguide.chacha.com/ questions via SMS
http://www.sumbola.com/ 'helps content creators deliver their creations to a global market'
http://studentoffortune.com/ homework help
http://www.content.de/ text (german)
http://taskarmy.com/ “hybrid between odesk and fiverr”
http://crowdmug.com/ requesting photos of places
http://www.asksunday.com/ personal assistant

Expertise & Knowledge exchange
One or more persons use their expertise to fullfil tasks.
http://10eqs.com Knowledge exchange (they structure the problem and have their own experts)
http://eclerx.com/ Sales and Marketing Services, Financial Services
http://starmind.com/ A bit like a bad quora with money
http://g2link.com p2p risk management
http://www.trada.com/ search engine marketing done by a crowd
http://boostctr.com/ search engine marketing optimization
http://www.taskus.com/
http://www.mavenresearch.com/ consulting
https://www.minutebox.com/ expert advice over video

Contestbased
There is a call and many people respond to it. There are different systems who gets a prize and what happens.
http://99designs.com/ designstuff
http://www.mykindacrowd.com /ideas & problems
http://www.hiretheworld.com - Creative Contests and 1 to 1 employment platform for Design, Writing, Programming, and Administrative Tasks.

Translation
Services specialized for translating.
http://mygengo.com/
http://www.tolingo.com/
http://www.lingotek.com/
http://www.smartling.com/
http://www.onehourtranslation.com/
http://strakertranslations.com/
http://www.speaklike.com/
http://tran.sl crowd sourced translation engine

Offline Tasks
Services that are built around offline tasks. From grocery shopping to repairing windows. Sometimes also online tasks.
http://taskrabbit.com/
http://agentanything.com/
http://gigwalk.com/
http://www.milk.ly/

Freelancer
Platforms which help to find the right person for a job. Sometimes they also help the client to have more control.
http://elance.com/
http://freelancer.com/http://guru.com/
http://odesk.com/
http://scriptlance.com/
http://vworker.com/
http://expertbids.com/ Professional services "get free personalized bids"
http://biddyon.com.au/
http://www.voices.com/ voice over jobs
http://www.canyoucode.com/

Fixed amount of money
These platforms only connect the client with the worker. There are easy tools to built such a platform, as a result there are many. Tasks are often stupid.
http://fiverr.com/
http://fourerr.com/
http://gigbucks.com/
http://gighour.com/
http://gigme5.com/
http://tenbux.com/
http://twentyville.com/
http://uphype.com/

Reviews and Opinions
The title says it all.
http://softwarejudge.com
http://epinions.com
http://reviewme.com
http://reviewparty.com
http://reviewstream.com
http://www.utest.com/

Spammy stuff
Many of these platforms could be listed in 'Simple Tasks', but I wanted to separate them because most of the listed tasks are only targeted towards spam. Like "follow me on twitter, like that page, submit your data to that site,...". (Sorry, if I put your company here. That is what I saw. I could be wrong.)
http://jobboy.com/
http://microworkers.com/ and http://minifreelance.com/ (not sure if just a copy or the same company)
http://rapidworkers.com/
http://simpleworkers.com/, http://minijobz.com/ and http://www.desklancer.com/ (look also the same)
http://shorttask.com/ Research, Data Entry, Writing Design

???
Not enough information on the website and I didn't test them already. Or they are just different from all others.
http://minuteworkers.com/
http://innocentive.com/ crowdsourcing ideas
http://www.little-favours.com/ (Facebook App, small stuff)
http://crowdengineering.com/ enterprise crowdsourcing solution with integrated community management
http://cumulusiq.com/ Knowledge as a service; SAP Integration
http://kaggle.com/ post data, let scientists analyze. competition style.
https://atizo.com/ idea crowdsourcing (german)
http://researchgate.net/ scientific crowdsourcing
http://www.poptent.net/ video contest
http://www.rippple.com/ funding, service and cutomers for startups provided by a crowd
http://castingwords.com/ transcription
http://store.exvo.com/teams
http://www.copify.com/ copywriting
http://www.imagebrief.com/ photos contest
http://www.speechpad.com/ transcription
http://txteagle.com/ sms based
http://world4brains.com/ crowdconsulting
http://www.go4customer.com/ outsourcing business solutions