UK Maps - London maps especially

Modern

STREETMAP http://www.streetmap.co.uk/
MULTIMAP http://uk.multimap.com/map/home.cgi?client=public&overviewmap=ap
Ordnance Survey http://www.ordsvy.gov.uk/
http://www.buzzlondon.com
Greater London postal districts http://www.city.ac.uk/accommodation/postcode.htm
Any-Villagesite http://www.any-village.co.uk/map.asp

Historic

ParLoc is indeed a very useful, free program, it can be downloaded:
http://www.dmbennett.freeserve.co.uk/ParLoc.htm
It  isn't, however, a map program, but a parish locator - one can type in parish names, find surrounding ones, distances between them, etc. It gives
OS map references, so can be used in conjunction with an online or 'real' map for visual reference.

Lost Streets

www.lost-london.inuk.com and http://members.aol.com/WHall95037/london.html

http://www.gendocs.demon.co.uk/lon-str.html

Old Maps

London 1900 Pocket Atlas   can find with other goodies on http://freepages.genealogy.rootsweb.com/~nmfa/genealogy.html

Wonderful maps of London  1746 1862 etc www.motco.com
Map 1827 http://www.bathspa.ac.uk/greenwood/home.html
Map 1859: http://www.ph.ucla.edu/epi/snow/1859map/map1859.html
Map 1889 http://www.umich.edu/~risotto/maxzooms/sw/swe910.html
Map 1899 http://booth.lse.ac.uk

http://www.village-atlas.com/

Some City parishes http://www.londonancestor.com/maps/maps.htm
6inch maps (all UK) 1850-90     http://www.old-maps.co.uk/

Comments by someone else "There's a superb on-line resource for historical maps started for genealogical research but very comprehensive -- at:
http://freepages.genealogy.rootsweb.com/%7Egenmaps/index.html
I first came across this link on the computers at the British Library Map Library -- a pretty solid recommendation"

London Street atlases and directories on CD http://www.archivecdbooks.com

Mainly selling maps - which you may want http://www.alangodfreymaps.co.uk -- http://www.village-atlas.com/

Mostly USA - David Rumsey collection http://www.davidrumsey.com/