My favorite actually is Coleman Seam Sealer, about $6.  It comes in a foam head type applicator, you just run across your seams.  

Though I have, I don't use the sprays anymore.  There's two reasons.

A lot of technology has gone into allowing tents to breathe.  Though they may get really damp due to a hard pulverizing rain, they dry a 100 times more quickly than a tent that can't breathe.  Seal an entire tent with polymer spray, might as well be camping under a sheet of plastic.  The rain won't come through but you will be living in condensate the entire time.  I only seal the seams and where I see pin light coming through by setting up the tent in sunlight before I go camping.  

Second, the sprays throw the stuff everywhere, looks ugly as hell, and very often end up pealing. Its not like they make that stuff in different colors to match your tent.  Regardless of type,  It's so thin unless you are very careful of temperature, it dries before it adheres to the surface well enough.  Remember tents are not meant to pass fluid through.  The roll on applicator, you can put a thin thick layer on the seams and its thick enough it will soak into the pours enough to stay on there a lot longer.  Its about the same principle as the old bees wax however it dries to the touch.  

I can think of nothing I enjoy more than sleeping in a tent during a steady rain or light storm. That being said, I can think of nothing more miserable than trying to sleep in a tent in a torrential downpour.  Over half a century of camping, I've learned a few things.  

First, tent placement is critical when it comes to weather.  I put my back to the prevailing wind, not my door and I like, if possible, a slight slant to channel water away.  If not possible, I light trench it away.  I like under a cover like a tree.  The leaves will catch much of a torrential downpours energy.  If I know I'm in for a torrential downpour so hard it will pulverize right through my tent wall, I put a light weight tarp over my tent with simply tent poles or use trees.  I say light because it doesn't have to be a tarp, lightweight nylon does fine.  The point is not stopping the moisture, its simply to reduce the force.  Things like tarps and plastic have their own issues with weight, water runoff, and condensation.  This overhead barrier, simply reduces the force so what passes is that very same mist you get from a naked tent which then condensates and rolls off your tent while you stay dry.  

Keep in mind even with a good tent, every time you open that door, you are letting water in.  There really is no way around it  Its especially bad if the door is into the wind.  This is why I like my tent on a slight incline.  That way I can leave an area for water to collect inside.  A tent is not suppose to be a boat.  That's an obvious statement but then I can't begin to tell you how many people I've seen set their tent up in what is for sure to be standing water after a good rain.  

Last but not least, I set my tents up with back to the wind, least surface, and secure them down.  In these days of free standing tents is so easy to not stake them, however though a tent does not make a good boat, it makes an excellent sail.  You wouldn't be the first camper to chase his tent as it rolled away in a storm filled with his gear.  

Hope this helps.

Tj