John DeChancie’s “Skyway” trilogy: Starrigger, Red Limit Freeway and Paradox Alley.
It’s not immediately obvious (not until part-way through the second book, really) that it’s a time-travel series, so I guess mentioning it here at all is kind of a spoiler. But I’m not sure how to get around that. 
Now Wait for Last Year by Philip K. Dick
I couldn’t stand “The Man Who Folded Himself”.
That said, William Sleator is a great author, and wrote one of my favorite time-travel books, “The Green Futures of Tycho”.
“Time After Time,” by Karl Alexander, is a great read. The movie is even better.
Times without Number by John Brunner.
I’ll put what I thought was interesting about it in a spoiler box because it kind of gives away the ending
The idea that time travel could never be because if there was time travel, someone would go back in time and change things over and over again until a reality that time travel never existed would be reached. And that one couldn’t be changed.
L. Sprague De Camp’s short story A Gun For Dinosaur is a truly fun read, like all his stuff.
Robert Charles Wilson’s A Bridge of Years.
A man in 1989 Seattle finds a tunnel in his basement that leads to 1962 New York City.
I’m surprised that no one’s mentioned Timescape by Gregory Benford.
NoClueBoy already mentioned it but Robert Silverberg’s Up the Line is one of the classic time travel novels.
Going with “straight” time travel, a great bet is Poul Anderson’s “Time Patrol” series:
Of the “Alternate History” variety, it’s hard to top H. Beam Piper’s standout classic “Lord Kalvan of Otherwhen.” in which a Pennsylvania State Trooper is accidently dragged into an alternate history in which a nasty theocracy holds power because it has the secret of manufacturing gunpowder (called “fireseed”) and by supplying or withholding the commondity it controls the feudal kingdoms of an alternate America. Of course, the trooper knows the ingredients and the proper porportions for it manufacture and also happens to be a student of the history of warfare…
The “Svetz” series, which Odinoneeye is describing here, are now in a book called Rainbow Mars, which consists of the five short stories mentioned above and a novella with the same name as the book set in the same series. The premise is interesting: it can be demonstrated, says Niven, that time travel is fantasy, not SF. Therefore, if somebody did build a time machine that worked, where would it go? The novella simply takes this concept over the top.
Recommended: The Complete Paratime (trade paper from Ace Books), which includes one short story based on a true event setting the scene for the rest, four stories featuring the Paratime Police that provide a background for the Lord Kalvan story, and then “Lord Kalvan of Otherwhen” itself, all in one volume. Not so highly recommended, but not bad, is The Great Kings’ War, a continuation of the Kalvan story which is IIRC by another author based on Piper’s outline.
Of mixed quality but classic Andre Norton SF adventure is the Ross Murdock/Time Traders series:
[ul][li]The Time Traders (1958) [/li][li]Galactic Derelict (1959) [/li][li]The Defiant Agents (1962) [/li][li]Key Out of Time (1963) [/li][li]Firehand (1994) (with P.M. Griffin) [/li][li]Echoes in time (1999) (with Sherwood Smith) [/li][li]Atlantis Endgame (2002) (with Sherwood Smith) [/ul][/li]I recall fondly reading the first four in my teens; I know nothing about the last three.
Well I am suprised that no one mentioned micheal crichtons(spelling?) “timeline” (which was made into a decent movie recently, but the book is MUCH better), which is a good book and offers an interesting technical perspective. Also if you have read “A Wrinkle in Time” you should certainly read the followup books “A Wind in the Door”, “A Swiftly Tilting Planet”, and “Many Waters.” The characters also grow up and mature as the story progresses, making this interesting. Her books are steeped very deeply in fantasy, which is a far cry from Crichtons “timeline” which attempts to offer a vbt of a theorotical scientific explaination of how time travel “might” be possible.
That book is on sale for CA$9.99 at a local bookstore. Hardcover too! I was thinking of getting it but I didn’t know if it was any good. I might pick up a copy.
Time Traveler’s Wife is quite good.
Another vote for Varley’s Milennium (and the movie with Kris Kristofferson) 
Tomorrow and Tomorrow by Charles Sheffield, which is not quite time travel in the way you may think of it, but is still quite good.
I read a book recently that was a retelling of/sequel to Wells’ Time Machine. I can’t remember the name. I’ll ask Ardred and get back to you. It was fantastic.
L. Sprague de Camp did quite a bt of time travel. His Lest Darkness Fall has already been mentioned, as has his short story “A Gun for Dinosaur” (written in response to Ray Bradbury’s dinosaur-hunting story “The Sound of Thunder”, which was parodied on “The Simpsons”). de Camp later wrote an entire series of stories using the characters from “A Gun for Dinosaur”, which were collected as Rivers of Time. He also wrote random short stories, like “A Gun for Aristotle”. And we can arguaby include his novel The Glory that Was.
Robert Silverberg’s Up the Line
Michael Moorcock’s Behold the Man (a short story, later expanded to a novel).
Leo Frankowski’s series about a time-traveling engineer.
Pepper Mill is a big fan of Diana Gabaldon’s stories
Robert L. Forward tried to write a time travel story consistent with modern physics in Timemaster
A Connecticut Yankee in King Arthur’s Court by Mark Twain is still one of my favorites.
The Time Ship by Stephen Baxter. That’s the sequel to HG Wells’ Time Machine. Very… far reaching… time travel.
Check out Edward Bellamy’s Looking Backward. I was written in 1888, and tells the story of a guy who goes to sleep and wakes up in 2000. It’s quite short, more of a novella, and a great read. It’s not science fiction as much as a rather obvious attempt to argue for socialism, but it was highly influential on later works dealing with utopian/dystopian and time-travel themes. If you’re interested in the genre as a whole, it’s a must-read.
The Guns of the South. By Harry Turtledove.
While time is not the major theme of the book…it is part of it. It’s also a great alternative history read.