I'm not sure if this website is entirely legal, and I hope I won't get kicked off the forum for it, but here's a website I recently found:http://www.sheetsearch.com/
You can download PDFs of lots of music, not just classical, but also popular music. It does have a whole lot of classical music, including scores by modern and living composers. It has some more recent music that can't be found on IMSLP.
There are a few weird things about the site - for example, music by Beethoven is found both under B for Beethoven and L for Ludwig van Beethoven. The site does have a search function if you're looking for a particular composer and you're not sure whether to look him up under his first name or last name.
But yeah, I think it's worth a look.