I just started reading Dies the Fire by S.M. Stirling. I'm really enjoying it.
Another vote for S.M. Stirling books. Most are generally quite fun reads, although with a bit of a pattern to them. All different flavors of alternate history. Island in the Sea of Time is a great book (and a great trilogy if you want to keep going). Takes a small, modern population (island of Nantucket, off of Cape Cod), and drops them into the distant past, as is.
"Dies the Fire" is the first book in another series, but plays off of the same event. This one stays with our world/timeframe, but all technology stops working (also screws with some chemistry, etc so that guns are out of the picture, essentially forcing people to go medieval/old school). Fun survival/rebuilding/end of the world series.
Ringworld by Larry Niven (and the sequels, if you like the original).
Another good one. Kinda odd in spots, and when you finish the series, you feel like you didn't really scratch the surface of the Ringworld, but that's only because it's so well built up, with all kinds of crazy history and backstory, and unbelievably massive to boot. Also, you get the Kzinti species involved in the story, so there's a Star Trek connection there as well. Although, in actuality, it's because Niven wrote that episode of TAS (Slaver Weapon), so there's plenty of carry over there, with Kzinti, Slavers, and the stasis boxes appearing in both places...
And a push for Hyperion and its sequel.
Hell, i vote for all 4. The Hyperion Cantos is a great example of well-done world building, and split into two books of 2, seperated by a few hundred years. That makes it a little more fun, because you get very into these worlds, and then get to go back to those worlds later and examine the differences after the big changes at the end of the first duology. The first 2 books are set up similarly to Canterbury Tales, with a group of very different people on a pilgrimage, telling their stories of why they are going to Hyperion.
This series won all kinds of awards, and turned me on to Dan Simmons' other works, so I'm a big fan.