The International Music Score Library Project is a cool site that lets you download public domain music scores for free:
MSLP attempts to create a virtual library containing all public domain musical scores, as well as scores from composers who are willing to share their music with the world without charge. You can read the full list of goals that IMSLP will try to achieve.
IMSLP also encourages the exchange of musical ideas, both in the form of musical works, and in the analysis of existing ones. Therefore, feel free to create/edit a page with your analysis of a particular piece (please use the “Discussion” link on the work page of that particular piece).
The site already offers over 17,000 scores.
Unfortunately, the “modern” category (post 1945) is woefully underrepresented, because of copyright laws.
I was able to download a PDF of Terry Riley’s In C, one of the seminal works of minimalist music, but only 69 composers are currently represented at all in this category.
If you’re creating music that uses traditional, or even non-traditional, notation, check out the IMSLP and consider sharing your scores via the site.