Huh. I just realized how much of the stuff I read and buy is basically really depressing for the most part of the story.
But a few lighter recs in the sf/fantasy genres, since you're partial to Pratchett already:
Connie Willis,
To Say Nothing of the Dog. A marvelous comedy of errors involving Oxford historians time-traveling to the Victorian era in search of the bishop's bird stump, which their funding-provider insists on tracking down. I'll warn you that the other stories in the same universe,
Fire Watch,
Doomsday Book,
Blackout, and
All Clear, are overall rather bleak until the end.
Freebie at the Baen Free Library: the first two books of Rick Cook's Wiz Biz series. It's got a computer programmer kidnapped to a magical world to help defeat evil, and while the first book,
Wizard's Bane is a bit on the serious side because of the defeating evil bit, the rest of the books in the series are almost pure comedy. NB: You'll have to pick up the 2nd book,
The Wizardry Compiled, at the old Free Library page directly on the Baen.com site, since the Webscriptions omnibus version seems to only contain the 1st book.
John Moore writes fun light fantasy that satirizes fantasy tropes. Pity it only seems like
Slay and Rescue is available as e-book, but it's fun if you've ever wondered how Prince Charming coped with being fixed up with all those princesses.
And Lawrence Watt-Evans'
Ethshar series is definitely mostly light-hearted. A few serious themes, but overall upbeat and often subtly comedic. Available in DRM-free MultiFormat from Fictionwise, which has a 50% off coupon currently in the Deals forum.
Another couple of Fictionwise MultiFormats are:
Anne Logston's
Shadow series (only the first two books available), which is about the adventures of an elven thief in a mixed human-elven city with mildly interesting interspecies politics. They're nothing really special fantasy-wise, but pleasantly-written mind candy reads. Pity about the unfortunate Poser-looking cover art.
In the same vein is
Don Callander's
Dragon Companion (only the 1st book available), about the adventures of a human librarian from our world dropped into a standard elves and dragons fantasy world where humans are legendary creatures, and making a place for himself with his awesome powers of RESEARCH!
For humorous supernatural,
Nick Pollotta also has a few titles up. The
Bureau 13 books tie in to some role-playing game, but they're fun light reads about a team of secret government agents saving the world from unspeakable horrors. He's also got comedies with Civil War zombies and Lovecraftian expeditions.
John Dechancie's
Castle Perilous series seems to have been very popular back in the day, and it's now available in e-book. I've only read a couple of them which I got from the used bookshop, but the basic premise is of an extra-dimensional magical castle which leads to many other places, and the wacky hijinx its inhabitants have as they get lost amidst or invaded by said many other places.
And of course there's always the silly scrapes of upper class twit Bertie Wooster which his loyal manservant has to extract him from in P.G. Wodehouse's
Jeeves books, some of which are available here on MobileRead. I've also generally found Evelyn Waugh's books to be, well… maybe not "light" in mood, but definitely with a non-weighty and satirical air for the most part.
I should note that if you want your reading to be simply light-hearted, but not so much light-minded, I'd boost up the recs for Willis, Watt-Evans, and Waugh, who incorporate "serious" stuff in a mostly non-downbeat manner, like Pratchett does.
The others are basically pure escape reading and while entertaining, may be a little more shallow than you're looking for.