I find a LOT of stuff at the SlickDeals.net forums, in the Freebies section. It takes patience, because a lot of the best ones are only available for a short window so you need to check regularly - otherwise the chance is gone by the time you see a post.
I know that I've posted quite a few of the ones that I've like best here, and you'll find that many others do as well. I think anything that can be of even a little value for trips or helping save money for a trip is usually posted here as soon as someone finds it.
There are MANY forums out there dedicated to freebies, samples, and deals. It's a matter of finding the ones you like best. And also use caution. If it sounds too good to be true it probably is, and if it IS true then if you have a little patience and watch the board where the info was posted people will start coming forward. If something requres too much work (especially anything involving money) I wait until enough people have tried it successfully with no problems before I even begin to consider it.