I keep several files with different quotes and other messages I like. The latest one I've added is by Allen Lee Harris:

"The wonder is not that there is so much darkness; the wonder is that there is any light at all."

Here's a couple more of my favorites:

"The greatness of a nation and its moral progress can be measured by the way in which its animals are treated." - Mahatma Gandhi

"Perhaps it is better to be irresponsible and right, than to be responsible and wrong." - Winston Churchill