In my presentations, I may have several slides which feature a quote from a famous (sometimes not so famous) individual in the field. The quote may be a springboard into the topic or serve as support or reinforcement for the particular point I'm making. A typical Tom Peters presentation at one of his seminars, for example, may include dozens of slides with quotes. "I say that my conclusions are much more credible when I back them up with great sources," Tom says in this post from May, 2005. (I talked about using quotes a few months back here with examples.)

Like everything else, quotations work best when not over done. Too many quotes or quotes which are too long may bog down your presentation. And of course, if your quote is inaccurate or completely irrelevant to your point, it may undermine your efforts in a big way.

Where to get quotes?
Personally, I do not search for quotations on the internet very often. I almost always get mine straight from material I have read directly. My books for, example, are filled with tags and pages full of my comments and highlighter marks. I sketch a star and write a note to myself next to great passages for future reference. It's kind of messy, but it works for me. Still, there may be occasions when poking around one of the quotations pages may be of help. So below I have listed a few of the many places you can check for quotations. I'm certain there are even better resources, so please let me know and I'll update this post to make it better.

Tom Peters' slides from his website
As Tom says "we post all my slide shows so attendees can go back at their leisure and recall the logic of the presentation and "steal" some cool quotes to use in their presentations!" If nothing else, a look at the sources in Tom's slides may point you to the original material for deeper research.
Tom Peters' "Top 41" quotes and other free stuff
Various PDFs from TP. Excellent.
Wikiquote
Quotes, proverbs, etc. in several languages (thanks, Pawel!).
Yahoo's list of quotation sites
One of the best places to start. You can search sites on Yahoo popularity or alphabetically.
The brainy quote
Thousands of quotes by thousands of authors. Search by topic, author, or profession.
Famousquotations.com
Good selection. Searchable.
The Quotations page
Over 24,000 quotes, 2,700 authors
Quoteland.com
Many categories.
Quotationreference.com
Searchable by author or subject.
Famous quotes and quotations
Searchable and organized in a few broad categories.
Quotelady.com   
Organized by category here.
Great-Quotes.com
Over 54,000 famous sayings, proverbs, and quotes.
Quotation resources by about.com
Different.
Bartlett's Quotations
11,000 searchable quotations from literature
Business-related
Not great, but may be some gems in there.
Stupid Quotes About Hurricane Katrina
By Daniel Kurtzman
Dr. Gabriel Robins' "Good quotations by famous people"
Interesting list by a CS prof.
Positive Atheism's Big List of Quotations
A lot of categories but especially philosophy, religion, politics...
World's best quotes in 1-10 words
I like this short list from Career Lab.
Zen Quotes
A few quote from the world of Zen
Using quotes effectively
A few tips from the Idea Bank

Note: The photo of the woman making tea in the sample slide above was snapped by me about a year ago in Kobe. (You may notice something a bit odd about it — extra points for anyone who can guess what it is...).