I decided to take up the post thread Jim started (which Greg already followed up on) and talk about graphic novels I pass around. Many of them are at least somewhat all ages but for clarification I'll start point out the books that can be safely given to younger readers (i.e. if you let your kids read Harry Potter these comics will be fine for them). I'm also sticking to graphic novels that don't require a large investment of money in multiple books - most of these can be found in one collected novel or stand alone without the following books.
1. Mouse Guard: Fall and Mouse Guard: Winter
The are awesome mouse adventure books with gorgeous artwork. I have lent them out to many people including adults and kids alike. I have yet to find someone who doesn't enjoy these books on some level. They also make great stories to read to you kids. I love these stories so much I'm running a Mouse Guard game! Even if the RPG rules are hard to follow at times :) I'd link reviews to these books but we've done so many I don't know where to start.
2. The Mice Templar: The Prophesy
Which leads me to Mice Templar. Also a mouse adventure book but with darker undertones. This book reminds me of watching The Secret of Nimh as a kid - especially the rats. Well the bad rats at any rate. I really want to read the next book but this one alone is qorth it for the really cool owl artwork. See my review here.
3. Egg Story
This book is awesome! Too bad Arielle lost it *cries* (just kidding Arielle!) Very kid friendly and funny as well.
4. The Rabbi's Cat
This book may be a bit over the heads of some younger readers as far as the underlying messages, but the overall story is pretty accessible to most ages. To be safe for more conservative parents I'd say this is a teen and up book. I have Lee to thank for both this book and my next pick. I haven't read any other books by this creator so I can only recommend this one for sure. I especially love this book because the story is from the cat's perspective. Read my review here.
I'm only recommending the first Persepolis book here although the sequel was interesting as well and you can buy them as one book now. I'd say this is more of a teen and up book as many younger children wouldn't know much about the historical context of this book or the cultural context and it's harder to understand without that knowledge. However it is a book that draws you into the life of the creator very effectively. It's a book that makes you think while still providing entertainment.
6. Action Philosophers
Young adult and up although I'd be fine giving it to younger kids (you know, my own kids if I had them - you have to be more careful with what you give to other people's kids). All of these graphic novels are amazing and you learn a lot about philosophy along the way. I have passed these out to friends and professors at my college. I had one professor use the Marx issue for his Marxist anthropology class. Check them out here.
7. Mage: The Hero Discovered
As Mirth was my favorite character I couldn't get into the second installment of this story but this first book is excellent. I read it when I was fairly young and really it's no more adult oriented than Harry Potter's later books. There are some tragedies and sacrifices but the overall story is beautifully written and the art is clean and accessible. The colors are gorgeous and I'd hand this graphic novel to pretty much anyone. Just a few months ago I lent it to my former boss's 14 year old brother and he enjoyed it so I know it should at least be fine for teens.
8. The Books of Magic
Ah, the book where I first fell in love with Neil Gaiman - and yet so many Sandman fans I talk to have never read this graphic novel. Ironically this book is why I refused to read Harry Potter for the longest time as I thought J.K. Rowling stole the idea for her books from Gaiman. This is a young reader and up book. It's pretty cool because it has the chapters done by different artists to fit the different themes followed by the plot. If you are a Neil Gaiman fan and haven't read this you are missing out - but I've lent it to people who have never heard of Gaiman and still loved the book.
I have a few more books I could mention but they are the books that's I'd only lend to specific people - or they are more adult. So I'll save them for another day!