Tesco Home Panels is run by Tesco (funnily enough!). When you register, they will every so often get in touch to offer you the opportunity to test one of their products for free, in exchange for completing a survey. They also have some surveys for which you get points instead of free products, which you can exchange for Tesco vouchers, once you have amassed 50 points.

Some people seem to be offered a lot of trials, while for others (like me) they're few and far between. Still, worth doing. It's linked to your Tesco Clubcard, so my guess is that the more often you shop at Tesco, the more often they'll offer you free products. But this is just a guess.