FTL (Warhammer 40,000)
In the grim darkness of the far future, traveling between the stars is still as a much as a pain on sublight propulsion as it is today. Thankfully the various races have figured out various ways around Einstein's pesky "impassable" speed of light.
The Warp Drive is the standard method of interstellar travel in the Imperium of Man, as well as for the (mortal) forces of Chaos and Orks (in whose hands they are called Warp Engines). It works by creating a rift into the Immaterium, an alternate dimension of reality which voidships can use to travel to other systems in a reasonable timeframe. Unfortunately, the Warp sucks for travel, as its various currents can throw you way off course (or outright make you never return), and the realm's natural denizens will tear you apart if you don't have a Gellar Field (Unless you're the Orks, then you just krump the gitz). The former makes Navigators a necessity for long range FTL travel, to the point said Navigators have to do incest just so that there's enough of them for the Imperium.
Humanity discovered Warp travel during the Dark Age of Technology, which greatly accelerated their expansion across the stars. Unfortunately this reliance would bite them in the ass during the Age of Strife, as the Immaterium would become untraversable and cut worlds off from each other - coupled with the outbreak of Daemons and Psykers, this did not end well. This cutoff problem was one of many reasons why Big E tried to create the Imperial Webway before a certain Primarch did everything wrong and the Horus Heresy saw him put on
a golden toilet *BLAM* the Golden Throne for over 10,000 years.
The Webway is the main method of travel for the Eldar (as well as their edgy counterparts and the clowns) and the Necrons (via Dolmen Gates) and was going to replace Warp travel for the Imperium if Magnus didn't fuck up. Essentially an ever-changing network of 'highways' through the Warp, it was originally created by the Old Ones before the War in Heaven killed them all - the Eldar essentially 'borrowed' the technology now that the Old Ones were no longer around.
The other Necron FTL method, and arguably the best in the setting, an Inertialess Drive isn't as much as a loophole for the speed of light as it is a direct "fuck you" to it. Essentially removing all inertia from a vessel, this allows the 'Crons to travel at ludicrous speed in the Materium, and make capital ships as nimble as fighters. However, as a Nercron voidship remains in the Materium, physical objects and Warp storms still pose a hazard to travel, so whenever one of these heavily impedes use they just opt to use a Dolmen Gate.
AKA a Gravitic Drive, this is the FTL method for everyone's favorite weeaboo space communists. Unlike a proper Warp Drive (which this tech was reverse-engineered from in the early days of the Tau Empire), these barely dip a vessel into the Immaterium and aren't at the mercy of its dangers. This also makes it far safer, at the cost of speed (while it does go FTL, a Warp Drive will still far outclass one). This 'dive' is achieved by creating the FTL equivalent of a hydrofoil - the field developed by the drive is wing-shaped to take advantage of the turbulent, apparently 'ocean-like' boundary between realspace and the Warp, stalling the transition long enough that they will reach their destination before they even touch the actual Immaterium. The drive has to maintain a certain angle of attack for this 'dive' however, as otherwise the ship will just plunge into the Warp and into the arms/tentacles of its denizens. These are used in conjunction with the sublight but still relativistic ZFR Horizon Accelerator Engine to propel the Tau's expansion into the galaxy.
A specialized bio-ship used by the Tyranids to travel between systems, essentially functioning as wormhole generators by creating warped tunnels of spacetime. Using these tunnels allow the Nids to (practically) reach FTL velocities, but these speed-boosting conditons break down in the presence of a strong gravity well (such as that of a star system). And since the Narvhal is the only bio-ship capable of creating these spacetime corridors and is completely defenseless on its own, its a prime target for any fleet seeking to trap a Tyranid fleet in a system.