These are some of my favourites quotes. Usually I use one of them as signature on my mails.
1. To attain KNOWLEDGE, Add things every day;
To attain WISDOM, Remove things every day.
2. "You ordered a first-rate article and you must be content to pay the price"
-- Joseph Clement (engineer, corresponding with his client, Mr. Charles Babbage)
3. "The definition of insanity is doing exactly what you did before all over again, but expecting different results this time."
4. Learn from yesterday, live for today, hope for tomorrow.
The important thing is to not stop questioning. --Albert Einstein