How do you wish to access the maps?
Tried-and-true paper?
Published in an atlas for each state?
From a CD, viewed on big screen and printable onto paper?
On-line, viewed on big screen?
On-line, viewed on mobile device?
The USGS maps mentioned by Ripio are the ones compiled by the federal government. Updated from time to time, but paper ones not always current.
Most of the federal agencies which administer public lands - forests, etc. - publish paper maps (folding) of their various regions. So, each national forest would have its own.
There is an on-line route to get to the USGS base information and download it.
In other areas, folks use the USGS base information and update it from time to time. In Washington State, "Green Trails" is such a source. They use a scale slightly smaller than the USGS, so pages are smaller; hiking trails (current routes within a year or so) are green lines. Other states likely have similar.
Several companies - DeLorme is one - use the USGS map bases, compile and print at a smaller scale; similar to what I've seen of OS but not quite as detailed.
"Topo!" is a CD-based product that gives you all of the USGS, as of the date the CDs were burned, at several map scales; versions for PC and Mac. Nice feature is that you can print your own map is an area you want is on the seam between several of the paper ones. There are some overlays that can be found, by hunting around, that put trail information (updated, sort of) on top of the Topo images.
The various google-related map backgrounds (access on large screen or small) have optional displays for roads, hiking, etc.
Any part of the western USA you have in mind in particular?
