• Amazon.com, Amazon.co.uk, Amazon.fr, Amazon.de [English/French/German] Well-known store for books, CDs and other things.
• Magnatune offers music for downloading. Listen to the entire piece for free, and if you want to buy it, determine the price yourself (within certain limits).
• Sheet Music Plus [English] Store for sheet music and other music-related accessories. They are located in California and ship worldwide.

• Themefinder has a database of mostly classical music, searchable with regular expressions.
• MusicBrainz (a music metadatabase) can be used to tag your MP3 tracks. See Eddy Elliott's MP3 Tagger, for example.
• Shazam is a commercial audio fingerprinting service from the UK, also available in other countries. With audio fingerprinting techniques, you can search for a recording, not for a melody. So, with Shazam or similar services you can reliably identify popular music if you can provide a sample of a recording that is in their database. If you only have a recording of someone else playing the same music, you are out of luck unless that's also in their database. Of course, you can also buy ringtones there, or buy CDs with the identified music.
• Tunatic - yet another audio fingerprinting (?) service.
• Tunespotting.com has a database that can be searched in three ways: using notes, by playing on the computer keyboard, and with text.
• MusicDNS is a free open-source audio fingerprinting system. Unlike Shazam, it seems to create just one fingerprint for whatever snippet of audio you give it, thereby ending up with different fingerprints for, say, the first 10 seconds of a track versus the first 20 seconds of the same track. Shazam, on the other hand, creates multiple fingerprints and therefore would create a small set of fingerprints for the first 10 seconds and a larger set for the first 20 seconds of the same piece, where the larger set is a superset of the smaller one. So, with Shazam you are largely independent of what exact snippet of a recording you have, while with MusicDNS you can only identify a recording if you have a whole track to create a fingerprint from.

MIRsystems.info lists and describes various Music Information Retrieval Systems for content-based searching.

• At musicnotes.com, it is possible to view sheet music and listen to a MIDI version for free. Sometimes, for example when trying to enter sheet music for a new melody into Musipedia, this is all you need.
• A huge source of sheet music is Musipedia's MIDI web search in combination with MidiNotate Musician. Although most MIDI files are meant more for listening than for viewing, MIDINotate Musician (available as a free 30-day trial version) does a surprisingly good job with turning them into sheet music anyways.

• Classic Cat - the free classical music directory [English] Directory of free classical music made available by the performers, sorted by composer and work. With biographical links for the composers.
• Orpheus.at [German] - Portal for Austrian, German, and Swiss music sites
• Klassieke-muziek.pagina.nl [Dutch] - tons of music-related links on one page
• Celtic Headquarters [Dutch] - folk music links

The Whistler [English] - whistling as an art

The Nobel Prize Internet Archive [English] - contains up-to-date information of all latest and past Nobel Prize laureates.

Spam protection: SaferSignup.com [English, German] - allows you to control who can send you e-mail by creating unique mail aliases for every communication partner.