I've just kinda tried to think of some I use regularly. I have lots, and a bunch that I'll probably never use again, but everything can add up.
Almost universal - Aeroplan/ Air miles. I have both an air miles card and a credit card that gives miles per $20 spent. In stores that accept air miles, I can essentially double dip! Air miles can be used for everything from gift cards and blenders, to flights and car rentals. It takes a long time to build miles (in my opinion) but it's money you'd be spending anyways, so getting free reward on top is great. Aeroplan works much the same way, but I haven't found it offered is as many places.
Online - MyPoints.
This is a new one for me, but there are a lot of places that will reward you for doing stuff you already do (like browsing the web). A lot of them ask you to watch videos or do surveys for points, which is easy to do in the background while doing other stuff. Usually points can be used toward gift cards. I've recently found MyPoiints because they give you points for printing coupons, and they have a TON of coupons (like currently 300 or something).
Gas, like PetroPoints/ EssoRewards
Again, you're getting gas and spending the money anyways, why not get some freebies on top? Gas reward points can be used toward discounts on gas or free items like car washes or candy.
Walmart credit card
So this is a new one, but a great one if you shop at one store all the time. A lot of places will have their own credit card that will either give you a discount or some sort of rewards points. Walmart's card gives you points everywhere you use it, but more points if you use it at Walmart, plus no annual fee. Walmart has been a great place to price match and use coupons, so it might be a worthwhile thing to get to get even more rewards.
Movies - Scene
Maybe you don't go to the movies that much, but when you do go you go with a group. How many of your friends have this card (in my group, like no one). You have the chance to use your scene card for everyone in your party, which might be the equivalent to free popcorn and such. They also have a Scene card in a credit card too I believe, which would give you points towards movies on everything you would be buying anyways.
Airport parking - Park n fly
This was a recent find that gives you points per stay, towards free parking later. If you have a CAA card you can also get great discounts on your stay in these parking lots.
Hotels - Marriot, Delta, etc
This is probably a very large category, but a lot of hotels (almost every one I've looked into) have some sort of rewards program. The ones that are kinda fun work at chains of hotels, wherever you go in the world.
Grocery - PC Plus/ Sobeys
Another broad category but lots of grocery stores give you points for shopping there. Doesn't hurt to ask a t the cash register if they have anything like this you could sign up for.
Brand/Food - Lumchmate club
Sorry mom, but I do still eat Lunchables. I'm not sure how many products have similar points for buying their product, but I know these guys do (I think Crayola might?). In saying this, it's taken me almost a year to get the points I have now.. Which is almost enough for a free lunchable. That's a lot of $3 Lunchables to get one free, but hey, I was buying the product anyways!