Here's a somewhat outdated list:

Bittrex: support@bittrex.com
c-cex: https://c-cex.com/?id=vote#suggest
Bter: https://bter.com/voting
Hitbtc: https://hitbtc.com/about-us
xBTCE: https://support.xbtce.info/Tickets/Submit
OpenLedger: https://bitshares.openledger.info
Liqui.io: support@liqui.io
exmo: https://support.exmo.com/hc/en-us/requests/new
Livecoin: https://www.livecoin.net/en/coin-listing
Bleutrade: https://bleutrade.com/voting
Cryptonarium: https://cryptonarium.com/en/support/
BloomBit: support@bloombit.net
Chameleon.bit: admin@chameleonbit.trade
alcurex: https://bitcointalk.org/index.php?topic=458732
SouthXchange: support@southxchange.com
Tux Exchange: https://tcx.freshdesk.com/support/home
Bittylicious: support@bittylicious.com