Google Maps now has cycling directions (beta)
Just click the bike icon.
It wants to route you along rivers and tow paths from what I've seen.
It also does a time estimate for cycling which is interesting.
The street view changes somewhat so you can see more detail but no road colour codes.

Wow - tried my commute route which it gets about 50% right. Time estimate is a joke - 14 miles = 1.5 hours!My actual route is 15.25 miles and takes me 50 minutes.

I had a look at this a few days back. It assumes you have an off road bike and are also prepared to risk the glass strewn badly surfaced cycle paths. Still it is a good idea, would be nice to have an option to stick to roads and not use paths or bridleways. But I suppose you can do that on the driving options.

Those times make you feel good though! Mine's 19 miles and takes about 58 minutes. Google maps has it as 1 hour 58.

It has come up with an interesting alternative for some of my commute which I'm going to try.

I am also wondering if it thinks about the gradient when it works out the routes?

Agreed on the mountain bike assumption. How often are those tracks going to be dry so you can get to work without mud splattered all over your clothes/ shoes etc?

I've been using Bikehub for cycle routes (and walking ones too!) for six months now and it's much more mature.

Google maps gets my commute wrong at the moment, although I'm sure it will catch up.

Bikehub has the following features and options:

Choice of busy, quiet or balanced routes.
Route can be displayed over 6 different map types.
Shows the elevation profile.
Shows the surface type for each instruction.
You can export routes to GPS.
Mobile apps allow downloading of map tiles so you don't need an internet connection on the move.

I sound like a bit of a salesman but it's all open source and free, and the more people who use it, the better it will get!

That's a really good site.
The elevation profile looks really handy for trying new routes.

Bikehub doesn't work on my work machine...