John & Anne Nuttall's website - a must for summit baggers
Alan Dawson's website - for up-to-date information about the Marilyns
Statistical Topics in Hillwalking - home of the Database of British Hills (DoBH)
Myrddyn Phillips interviews - Myrddyn Phillips's YouTube interviews on hill bagging and related topics, including interviews with Graham Jackson from the DoBH survey team, and John and Anne Nuttall
Ordnance Survey - Britain's national mapping agency
Streetmap - excellent source of maps in digitised format - I obtained most of my background maps for this site from here
Multimap - another source of digitised maps
GPS Utility - GPS Software to download - even the free unregistered version is good
Garmin - GPS equipment manufacturers
TACit Tables - source of definitive lists of hills in Britain
Scottish Mountaineering Club (SMC) - publisher of Munro's Tables and other Scottish hill guidebooks
Phil Newby's GPS site - waypoints in all sorts of formats, as well as lots of other GPS stuff
UK Rec Walking Discussion Group - lively online discussion on walking related topics
RHB Discussion Group - for Marilyn bagging and other hill-walking related topics
UK Walking Forum - Walking and Hiking Forum
Mad about Mountains - Mountain info and photos, and descriptions of walks to Marilyn tops
Been There - Done That - An "unofficial guide to Great Britain"
Where's the Path - walking diary of some fellow Marilyn and County Top baggers
Lakeland Walker - walking diary of a Wainwright bagger
Welsh 3000s Challenge / 14 Peaks / 15 Peaks
Wilderness Wales - lots of photos and other info on Welsh mountains
Morgs 4 Mountains - hill-walking site with nice photos and trip reports
The Pennine Way Site - good source for Pennine Way info
Simon's Mountain Lists - hill list history, photographs, and discussion on hill-bagging
Summit Journeys - Mountain walking and rock climbing holidays and courses
Mountain Views - Hillwalking Resource for Ireland, with similar aims and features to this site
UK Prominent Peaks - Prominence based hillwalking challenges
- Online guide to Lakeland's finest valley and surrounding area
The Mountains of England and Wales volumes 1 & 2 by John & Anne Nuttall - Cicerone Press, 1989 & 1990 - ISBN 1 85284 036 6 & 1 85284 037 4
The Relative Hills of Britain by Alan Dawson - Cicerone Press, 1992 - ISBN 1 85284 068 4
A Pictorial Guide to the Lakeland Fells books 1-7 by A Wainwright - Westmorland Gazette & Michael Joseph Ltd, 1955-1966
Mountain Tables by Michael Dewey - Constable & Co Ltd, 1995 - ISBN 0 09 474520 X
Complete Lakeland Fells by Bill Birkett - Harper Collins Willow, 1998 - ISBN 0583322093
The Mountain Summits of England and Wales by Chris Buxton & Gwyn Lewis - Red Dial Publications, 1986 - ISBN 0 9511723 0 1
The Mountains of England and Wales by George Bridge - Gaston's Alpine Books & West Col Productions, 1973 - now out of print