ETA (subclass 601): Available to passport holders from the USA, Canada, Singapore, Hong Kong, Malaysia, South Korea, Japan and others. Apply via the official Australian ETA app.
eVisitor (subclass 651): Free, for EU member states and a handful of others (Iceland, Liechtenstein, Norway, San Marino, Switzerland, Vatican City, UK).
Visitor visa (subclass 600): For longer stays or nationalities not covered by the ETA/eVisitor systems. Costs from AUD $195.
Always apply through immi.homeaffairs.gov.au - third-party sites often charge unnecessary fees.