Telstra
Fastest network (NextG 3.5G), best coverage by far, generally the most expensive by far
Optus
No.2 to Telstra and heavily reliant on its mobiles business, so lots of product offerings.
(coming soon)
Vodafone
Another step down in performance/coverage but generally comparable to Optus. Often some great deals.
AUD$79 3G modem with 2Gb usage; $29 recharge expires in 30 days; recharging allows unused data to be hoarded up to a total of 14Gb
(coming soon)
Three
Hutchison (three) merged with Vodafone in their network and operations, so is almost the same as Vodafone in many aspects, but sometimes has cheaper deals.
(coming soon)
Virgin Mobile
Uses Optus network; lowish cost
(coming soon)
Low-cost/high-use/no-support geek favourites:
exetel.com.au
pennytel.com.au has a phone SIM plan with v. cheap rates
Where to geek out the details:
whirlpool.net.au forums