I do mystery shopping for a few sites - www.retaileyes.co.uk, www.cybershoppers.nop.co.uk and www.grassroots.com. There are a number of others if you search on google. Each site seems to specialise in different types of mystery shopping (or at least, the jobs that appear for me appear that way!), so it is worth joining a number of them. Some sites are easier to become a member of than others, but on the whole I would suggest that you dont pay for membership, there are plenty of free ones out there (haven't joined biotrax, so can't comment on that). As meltoadhall says, you need to be very observant and sometimes the paperwork to complete is quite extensive. The money isn't particularly great, but if you can do a couple of jobs on the same trip, it is more worthwhile.
I also do surveys on ciao.co.uk, yougov and the lightspeed panel, they all pay a little or enter you into competitions - but no spam.
Good luck!