Good bits on the Baen Free Library:
1632 and its sequel, 1633. Eric Flint and David Weber dump a West Virginia mining town, ca 2000 AD, into the middle of Germany at the height of the 30 Years' War. Highly acclaimed historical AU. Ring of Fire and volume 1 of the Grantville Gazette, both short-story collections, are also here.
Oath of Swords, by David Weber, is the single best Paladin story I've ever seen. Bazhell Banakhson is a Barbarian Prince who's stuck in a bad situation when the God of War descends and asks him to become a Champion. Yeah, you can guess where he tells Him to stick the offer.
At least one of its sequels (The War God's Own) is also available here.
An Oblique Approach, In The Heart of Darkness, Destiny's Shield, and Fortune's Stroke are 2/3rds of the 'Belisarius' series by David Drake and Eric Flint. Warring factions from the Far Future have sent back technical advisors to the 5th century AD. One is backing an empire of terror in India... the other is building a bastion of freedom in Imperial Rome.
A Desert Called Peace is the first volume of Tom Kratman's Not-Really-911-Revenge-Porn series. VERY YMMV due to blatant political preaching, but well-written.
If you don't like conservative/libertarian political preaching, avoid Freehold, too. If you can handle that, though, it's good.
Retief! is the story of a diplomatic functionary in a rather imperialistic future. It's caper stories as much as anything else, and frequently hilarious. Highly recommended.
The Excalibur Alternative is a sequel to David Drake's "Ranks of Bronze", with a group of medieval warriors kidnapped by aliens to serve as enforcers on primitive planets so they don't violate their version of the Prime Directive.
The Shadow of the Lion - Mercedes Lackey, Eric Flint, and Dave Freer. First of a series of Magical History AU, with the basic premise that the Library of Alexandria was never destroyed and the knowledge of magic never lost. In medieval Venice, political intrigue is rife... (Readers of C J Cherryh's "Merovingian Nights" shared-world series will note that Lackey has reused several plot points from her contributions there.)
March Upcountry, March to the Sea -- first two of four in Ringo and Weber's coming-of-age tale, with the bratty, spoiled Prince Roger and his bodyguards marooned on a primitive planet.
The Wizardry Compiled - What happens when you let a computer programmer loose in a world where magic works? Well, first, he gets into a whole lot of trouble. Then he gets to work...
Earthweb - Earth is threatened by Berserker-like monsters. Defense planning takes the idea of cloud-sourcing to its ultimate conclusion.
Starliner is a David Drake classic, with the troubled staff of an intersteller cruise liner dealing with crisis after crisis and trying to protect and please their passengers.
Inherit The Stars is possibly one of the best. A classic from the 70's, it tells the tale of a genuine scientific investigation, with strong character-based action. Explorers on the Moon have found a human corpse in a spacesuit... that has been carbon-dated to 50,000 years ago!
Mutineer's Moon by David Weber starts with an astronaut testing a new sensor with a flyby of the far side of the moon... only to be caught up in an intersteller mutiny that started 50,000 years ago.
--
Sucrose Octanitrate.
Proof positive that with sufficient motivation, you can make
anything explode.
)