Australia, New Zealand & the South Pacific - Melbourne-Adelaide car rental

Other than the big U.S.-based chains + Delta and Europcar, are there any agencies I am missing?

Try www.standbycars.com.au / www.standbyrelocs.com - they're a consolidator, and also consolidate relocs of cars / vans.

You've potentially missed Budget and Thrifty.

Best deals right now from top of my head:

Budget 4 days for the price of 3 - you'll see it on their homepage (http://www.budget.com.au/)

I usually find one of these 2 companies to be the cheapest, but all depends on what you want and where from really.

Also keep your eye on BuckScoop (http://www.buckscoop.com.au/) which gives details of car rental deals when they come out.

Good luck!

A few more to try
https://vroomvroomvroom.com.au/ A consolidator of the majors
http://www.redspotrentals.com.au/v2/ 2nd tier car rental company and at some airports