The Next Generation Onwards
Plots involving the Enterprise traveling beyond warp 10 were once in the original series (such as warp 14.1 in That Which Survives), but for The Next Generation it was decided that these would no longer be featured. A new warp scale was drawn up, with warp factor 10 set as an unattainable maximum. This is described in some technical manuals as Eugene's Limit, in homage to creator/producer Gene Roddenberry. Normal maximum warp in the original series was warp 8.
The limit of 10 did not entirely stop warp inflation. By the mid-24th century, the Enterprise-D could travel at warp 9.8 at "extreme risk", while normal maximum operating speed was warp 9.6 and maximum rated cruise was warp 9.2. In the episode "Where No One Has..." the Enterprise-D was shown to exceed Warp 10, traveling a billion light-years from their home galaxy in a matter of seconds. The Intrepid-class starship Voyager has a maximum sustainable cruising speed of warp 9.975, the Enterprise-E can go even faster at Warp 9.985. In the alternative future depicted in "All Good Things..." (the final episode of the Star Trek:TNG), Federation starships travel at warp 13, although this is most likely due to a likely restructuring of the warp scale, perhaps putting warp 15 at representing infinite velocity.
Read more about this topic: Warp Drive