I was listening to FLL ATC and I heard the callsign “Yo” which was for Yellow Air Taxi ( took me a while to figure it out). They also had “Cactus” for America West and “Citrus” for Airtran. I’ll also be interested in others you may have heard.
FAA Publication JO 7340.1Z contains a list of all call signs.
Scroll down the page to find the publication and then open it up (HTML format) or download (PDF) to your drive. You’ll want to read chapter 3.
Also, from the main “Live Tracking” page at FA, you can select the “View All” option for airlines. The airline list will include the common callsigns for most of the airlines.