Here's a number of off-road routes I've created. They're all based off MVUM maps (Motorized Vehicle Use Map), and designed using a combination of MVUM, topographical maps, Google Maps, and Google Earth.

I won't go into great detail about MVUMs, but know that you are required to carry the map with you while utilizing the roads in a National Forest. As far as I understand the rules/laws: If you carry just a section of the map, you're required to also carry any informative parts of the map (such as the road restriction indexes, various tables/charts, etc).

Refer to the National Forest web site (or visit the ranger station) for more info, and to acquire maps. Far as I know, you can get printed MVUM maps free at any National Forest Ranger Station. The maps are available for download (free, on the NF websites) and are in PDF file format.

Some MVUM pdf maps are very well presented (as letter size (8.5"x11")), such as Olympic National Forest's maps. While other National Forest pdf maps are huge (>40" x >40"). I use Foxit Reader (free pdf reader) and GPStill (pdf utility program). GPStill will easily slice up a large pdf into smaller sections for printing (tutorial for slicing up large pdfs is available on GPStill's website).

Other resources:
Struggling with getting map tracks/waypoints into your GPS? These articles might help:
Google Earth to GPS info. From the same author, is a newer routine. However, read both and see which you might like better.

National Forest Sites:
Mt. Baker-Snoqualmie National Forest Homepage :: VMUMs
Coville National Forest Homepage :: MVUMs
Olympic National Forest Homepage :: MVUMs
Gifford Pinchot National Park Homepage :: MVUMs
Warning!
Road closures and restrictions can change at any time! Always check with the National Forest you visit prior to hitting the trail for the latest information! The maps here are not 100% infallible.

TREAD LIGHTLY. These routes are not hard core 4x4 "ORV" areas, these are National Forest roads. Thus, they deserve (and require) everyone's utmost respect. They are our roads!

Permits are required in National Forests! As well, you must possess the MVUM map while accessing National Forest roads (or sections of the map and its table of contents/indexes/closures/etc)

Refer to the National Forest website and/or Ranger Station(s) for further information (permits, closures, dispersed camping info, etc). Also make sure you check for addendum/updates for the MVUMs, these are living documents and are subject to change!

Understand the MVUM, and use it while traveling as the reference. Not every road you find in a National Forest is authorized for motorized vehicles. Refer to the MVUM when in doubt, and if still unsure just don't go down that road (oh, pun).
Notes about my Google Maps:
--Any route/track in the below maps with an obviously red tone are seasonal routes and thus restricted part of the year.
--Any route in yellow are probably roads that don't show up in google map (I say "probably" because sometimes I just pick yellow as a color and it very well could be a displayed road in google map too). However, these roads are in the MVUM and most likely also show up in satellite imagery and/or in topo maps. Existence in the MVUM trumps all, however 99.9% of the time I will cross reference all routes I create with at least satellite imagery and/or topo maps.
--Google Maps seems to have quite a few National Forest roads mis-labled. Always refer to the MVUM for road number designations.

I suggest clicking the link below the map to open the map up in google maps proper -- and then be able to see more info, show the map in google earth, etc.
Long east-west routes along the southern end of Olympic National Forest.
Three suggested options:
~70.7 miles (Main Routes Loop)
~99.9 miles (Main5-->Main4-->Extended routes)
~104.7 miles (Main-->Main2-->Main3-->Extended routes)
MVUM(s) used: Oly.NF (both Pacific and Hood Canal Ranger Districts)

View MVUM - Southern Olympic Nat. Forest in a larger map
In the footsteps of Mt. Rainer
38+ mile loop (with optional side tracks)
MVUM used: Mt. Baker/Snoqualmie NF

View MVUM - Footsteps of Mt. Rainer in a larger map
Snoqualmie/Lester Loop
Approx. 45mi loop. Note that there's a gate by Lester, so entering/exiting there is most likely a non-option.
MVUM used: Mt. Baker/Snoqualmie NF
Note:I keep having issues with the imbedded map centering correctly here. If you don't see the routes, just zoom out a bit, it's most likely just to the NE of the current view.

View MVUM - Snoqualmie/Lester Loop in a larger map
Hoodsport<-->Brinnon
MVUM used: Oly. NF, Hood Canal Ranger District

View MVUM - Oly Nat. Forest, Hoodsport-Brinnon in a larger map
Leland<-->Quilcene
~33mi loop
MVUM used: Oly. NF, Hood Canal District

View MVUM - Oly Nat. Forest, Leland-Quilcene in a larger map
Lake Cresent--Forks MVUM used: Oly. Nat. Forest, Pacific Ranger District

View MVUM Solduc-Cresent Lk. in a larger map