Here's what we use... http://www.amazon.com/SanDisk-Class...F8&qid=1363965827&sr=8-3&keywords=sandisk+2gb
Sandisk, same as Dan. There shockproof an tempproof. I just picked up two 4gb from staples for 12$. Hell of a deal considering their normally $19.99 each
Remember to get the proper speed class device. Recording HD video or multiple shot High res stills need faster cards. Usually is a recommendation in the manual.
Glad to see this posted because I knew I had to go to the store this weekend for something but couldn't remember what. I use the same ones Dan posted. Have to go pick up a few more of them.
I would recommend with whatever cards you get. To get 2 new ones for each camera label them to said camera and only switch those 2 out with that camera. That will help to avoid any SD card issues in the future. It isnt as crucial if all your cameras are the same brand but still a good idea.
If your trail cam is HD I can fully recommend one of the Sandisk Ultra SD cards. I've been using one in my Galaxy S III and it allows my fast burst camera app to store about 5 more pics/second than the regular card did. Also it does a real nice job with 1080p HD video recording.