It struck me that it would be useful to put together a bunch of links for online trail maps all in one place.

So, I'm working on it. I'll add more as they occur to me, and as always I welcome suggestions:

Bear Brook State Park, Allenstown: http://www.snemba.org/bearbrooktrails.jpg 

Beaver Brook, Hollis: http://www.beaverbrook.org/PDFs/bbmap.pdf

Cohos Trail: http://www.cohostrail.org/

Dunstable, Mass: http://web.mac.com/leedingle/iweb/drlt.org/Overview%20of%20Dunstable%20Conservation%20Lands.html

Fort Rock, Exeter: http://www.nemba.org/ridingzone/Fort_Rock.html 

Gap Mountain (Troy, NH): http://www.beqbooks.com/gap/gap.htm 

Great Brook Farm, Carlisle, Mass: http://www.mass.gov/dcr/parks/trails/great-brook600.gif 

Horse Hill Nature Preserve, Merrimack NH http://www.hhnp.org/TrailMaps.htm 

Groton (MA) Town Forest: http://www.townofgroton.org/xml/town/trails_committee/maps/town_forest.htm 

Massachusetts State Parks: http://www.mass.gov/dcr/parks/trails.htm

Metacomet-Monadnock Trail: http://amcberkshire.org/mmtrail

Mid State Trail (MA): http://www.midstatetrail.org/maps.htm

Middlesex Fells (off I-93 in Mass): http://www.mass.gov/dcr/parks/metroboston/fells.htm 

Mount Monadnock: http://www.nhparks.state.nh.us/pdf%20Maps/monadnockhiking.pdf 

Mountain biking Southern NH: http://beanroad.blogspot.com/2007/08/mountain-biking-new-hampshire.html List of smaller, local trails 

Monadanock Sunapee Greenway: http://www.msgtc.org/index.html

Nashua River Rail Trail: http://www.mass.gov/dcr/parks/images/nashCampMap.gif 

New Hampshire State Parks: http://www.nhtrails.org/Trailspages/HikingPages/HikTrails.html

Pack Monadnock/Miller State Park: http://www.nhparks.state.nh.us/pdf%20Maps/MillerOnPakkMonad.pdf 

Pawtuckaway State Park NH: http://www.nhparks.state.nh.us/ParksPages/Pawtuckaway/Pawtuckaway.html 

Pepperell (Mass.) Conservation Commission (no maps, but lots of info on several small, but nearby trail systems): http://www.town.pepperell.ma.us/conservation/Land.html

Souhegan Valley Land Trust: http://www.svlt.org/Trails/Default.asp 

Southern NH NEMBA trails: http://www.snemba.org/trails.html

Sunapee Ragged Kearsage Greenway: http://www.srkg.com/

Townsend (Mass.) State Forest: http://www.nemba.org/images/maps/twnsndsf.gif 

Wapack Trail: http://www.wapack.org/