Most, if not all, modern maps are based on the Ordnance Survey.
Many different road maps and guides of the UK are published annually which contain a index or gazetteer. They will not show smaller features in Hertfordshire but are useful if visiting the area by car - or for getting to grips with the overall geography of England.
Phillip's publish street atlases of many UK counties, including Hertfordshire. These have an index for the towns and larger villages, and also for streets - but omits to index small non-street features such as the farms which are marked on the map.
For detailed maps which show geographical features and individual farms, etc., and which are invaluable when visiting (or trying to visualise) rural areas, the best purchase are the Ordnance Survey Explorer maps.
For online maps which can be zoomed to street level see Multimap or 192. However the recent appearance of google maps, which is very user-friendly and is integrated with satellite photographs must be the first place to look.
It should be noted that county names and boundaries are not immovable and that modern maps show modern boundaries and names while most family history research uses the 19th century and earlier boundaries. There have been a number of comparatively minor adjustments to Hertfordshire's boundaries. See The Importance of Understanding Administrative Boundaries
Page revised November 2008