For practicality: Whatever it is that The Nox do in the Stargate TV show. It's not well explained because, well, no other race is advanced enough to understand it. Something about briefly causing two distant points in space to touch. Instantaneous travel to anywhere.
For impracticality: #1 The ring network in one of Stephen Baxter's novels. Kind of like the eponymous rings in the better known Stargate franchise, but the ring source and destination are fixed and transport time between rings is light speed, so you arrive years after you enter. And IIRC, you come out as an approximation of what you were when you went in. A very good approximation, but still an approximation. The advantage is that the journey seems instantaneous to the traveller.
#2 Whichever story has it that travel in hyperspace / subspace turns out to be slower than travel in real space. This may have been a throwaway Internet joke, but it still amuses me.
#3 Stephen King's Jaunt.