Free, legal access to millions of musical scores and parts.
Forum-style website for all things concerning the trumpet. Its “marketplace” page often features good quality used instruments and mouthpieces for sale.
***Take some of the advice given there with a grain of salt, however, since anyone can post there.***
Free metronome for practicing purposes.
Free reference website that contains concise, accurate entries for many musical terms. Also has brief entries for different composers and instruments.