As a glider pilot (as well as a PPL) I'm interested in getting forecasts not only accurate enough to decide if it is safe to fly, but also good enough to enable me to plan cross country flights of up to 200 nm without ending up in a field.
A good starting point is WeatherJack's site
http://s214580749.websitehome.co.uk/
It has links to pretty well all the weather sites that you could ever want. If you select Weather Page, from the home page, then RASP/blipmaps and scroll down you will find all the soundings (Tephigrams) that you are ever likely to need. WeatherJack also provides a tutorial so that you can understand them without needing a PHD in met.
I also subscribe to a German forecaster at
http://www.topmeteo.eu/go/home
This gives me today plus the next three days with a resolution of hourly, every two hours or every three hours depending on budget. They provide different data for aircraft, gliding, paragliding and ballooning. You can chose the geographic area(s) that interest you. If you don't subscribe then you can buy individual charts.
The "Charts GFS (American)" link from WeatherJack is interesting - it gives forecasts much further ahead than most others.