Here's my mega list. It hasn't been updated in a few years so some of them may have changed:
Market Research for you (currently oversubscribed) – www.marketresearch4u.co.uk
Research Helper (Leeds only)
Odesk – www.odesk.com
Code 3 Market Research – www.code3research.co.uk/
ETHNOS (open to all non-white people in the UK) www.ethnos.co.uk/
Select Research www.select-research.com/
MRFGR http://www.mrfgr.com/
The Research Link
Feedback Research http://www.feedbackresearch.co.uk/
QED Studios (does market research and lots of other stuff) http://www.qedstudios.com/
Interiority recruitment http://www.interiority-recruitment.com/
Gillian Kenny Medical Research
http://www.gilliankenny.com/Registration/Patient/
Bristol Focus (Bristol only!) http://www.gilliankenny.com/Registration/Patient/
Indiefield http://www.indiefield.co.uk/ (have not registered with yet)
Field Initiatives http://www.fieldinitiatives.co.uk/
Focus 4 People www.focus4people.com/ (EXTREMELY active in the London area!)
Volunteer 4 panels ww.volunteer4panels.com/
Free Index – (Market Research Search Engine) http://www.freeindex.co.uk/categories/advertising_and_marketing/market_research/
FHIOS Group www.fhios-uxd.com/
Quality Eye www.qualityeye.com/
Focus force http://www.focusforce.net/
Campden http://www.campden.co.uk/
Claret Market (London / Manchester ONLY) http://www.claret-uk.com/
Field facts http://www.fieldfacts.com/english.html
Focus pointe global http://www.focuspointeglobal.com/pages/fpg_Home.aspx
The focus network (WORLDWIDE) http://www.thefocusnetwork.com/
The focus group UK http://www.thefocusgroup.org.uk/
Research Opinions (Birmingham, Manchester, Leeds only) https://www.researchopinions.co.uk/
HC Recruit (???)http://www.hcrecruitment.co.uk/
Schlesinger Associates (US & UK ONLY!) http://www.schlesingerassociates.com/joindatabase.html (had a lot of difficulty with this online form for some reason!)
Response UK http://www.response-uk.co.uk/
Clix Research & Ayton Moon (no pay but free products to try!) www.clixresearch.com (worldwide)
Bunnyfield http://bunnyfield.com/
People4U www.people4u.org.uk
Focus Finder www.focus-finder.co.uk
The patient’s voice http://www.thepatientsvoice.org/
If you're in the west midlands, Podengo, code 3, and saros are good.
Most of the above are for London but they do online ones too.
I'd highly recommend using your alternative e-mail for this. I have an e-mail just for signing up for things and I use that e-mail. You'll get 10-20 a day!!