I use Avbrief (TAFs, METARs, F215, synoptic charts) and for same-day flying it is usually all one needs.
This one
http://www.arl.noaa.gov/ready/cmet.html
is my favourite for all weather beyond the "official data" carried by Avbrief and others, and allows a longer range view.
The Sembach site looks great but the low level cloud images are, I have usually found, too inaccurate to be relied on for being VFR. They regularly forecast no cloud at all below 7500ft when in reality there is a solid overcast at say 4000ft. This may be OK for the UK but is no good for e.g. France where one has to pick specific levels to avoid the military airspace that covers most of France.
BEWARE because some of the countless "amateur weather" websites carry old data which, presumably, they lift off other sites. On TAFs, the preparation date/time is in the TAF but this isn't the case for synoptic charts and you have no idea how old the chart is. It could be last year's. I have even had some WAP website deliver TAFs that were 3 days old; stopped using that one immediately.
This "lifting" of weather data is why the NOAA site above now has a device which prevents scripted downloading of the data.
I would very much like a website which can take GFS data and do a plot of icing and cloud tops en-route for a specified route. That is what I find is really missing from available info. OTOH it isn't possible to forecast accurately.
One can get an idea of icing levels from Form 214/215 but like all the "official" data that's same day only. The NOAA site has a way of getting the temperature profile but doing this for a long route is very laborious.