I don't know of a website. But there are freeware programs, like Finale's NotePad. For the most part, free programs allow you to enter a single instrument, and you can only have one voice - no bass note held while other notes are played above it, etc.
You could also use something like TuxGuitar, GuitarPro, or PowerTab - I think all of those allow the output of standard notation. I haven't used any of those much (I use the full-blown Finale), but my sense is they have some issues... you might never run into them if you stay in simple keys and don't transpose much.
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