Garmin Geko 200. Same size as mobile phone and has a bar-mount (although it works fine in Map-case of bar-bag and it totally waterproof).
Here it is on my accessory bar
http://www.geocities.com/freddered1/bars.jpg . You can just make out the Arrow pointing the way to a waypoint. It points directly at a waypoint and also displays the (constantly changing) distance to it, this means you can tell in advance of an upcoming waypoint/turn. When you arrive at the waypoint it then points you to the next one, and the next one...until you reach the end of the route (that you plotted on your separate Map software).
I use Fugawi (Where the Fugawi?) Map software 1:50,000 OS on my PC to plot a route.
Upload the route to the Garmin and it points the way (along with about 100 other features like keeping a constant track of where you have been that you can download to PC later, ETA, altitude, Av Speed moving, Av speed overall.....etc..etc).
It's one of the best pieces of kit I ever bought, does exactly what I wanted. Doesn't replace a map (because it does not have a map display) but it saves a lot of time when following a route, it means you can keep moving quickly on country lanes without the usual stopping at every unmarked junction to figure out where you are.
Beware !! It's totally useless for cycling (in my opinion) without PC Digital Maps (but brilliant when used in conjunction). Beware, the cheaper Geko 100 can not synch with PC so it (in my humble opinion) useless for cycling.
Geko 200
£85 from Comet
£5 for PC synch cable off Ebay
Mapping s/w is epensive but Fugawi 1:50,000 Ordnance Survey, although not perfect, works well for me and is about 40 quid for a huge area (much cheaper than buying OS Landranger Maps for the same area and you can print off selected areas). You can also 'Fly'through the OS map in 3D Terrian view..check out those big hills in 3D.
Great toys, highly recommended.