Sound logos for various things: http://en.wikipedia.org/wiki/Sound_trademark
Intel.Inside
NBC http://web.archive.org/web/20070307110031/http://www.uspto.gov/go/kids/soundex/72349496.mp3
AT&T http://online.wsj.com/article/SB10001424052970203718504577182951405815364.html
THX deep note:
http://youtu.be/eBlD2N_AwgI
Nokia tune http://en.wikipedia.org/wiki/Nokia_tune
Mac startup sounds:
http://youtu.be/xNgl_MjbAsM
Massive index of boot sounds:
http://titan08.free.fr/Boot%20Sounds/
Royalty Free Audio Logos: http://www.premiumbeat.com/royalty_free_music/audio-logos