Save $300 on telephone calls (more/less depending on your current plan & usage)
I am on the NeoSaver plan that for $4.95 per month provides pretty impressive value - it includes a DID (standard Australia telephone number anyone can call), $0.10 untimed calls to anywhere in Australia and many countries overseas (ie call someone in the USA and speak to them for hours and it just costs you ten cents), $0.12 per minute calls to mobiles charged per second and with no flag fall. The $4.95 includes $6 worth of calls (60*$0.10), the DID, and 20 SMS messages via their web portal.
Personally we have a standard Optus land line service, but even though we don't make many calls I've calculated that over the past couple of years we save between $25 to $30 per month by using MynetFone. Our biggest saving is through calls to mobiles. On a normal Optus plan calls to mobiles cost $0.40 per 30 seconds with a $0.35 flag fall. So a five minute call to a mobile from our landline costs $4.35 - the same call via MynetFone costs us $0.60. Make ten calls to mobiles over the course of a month, averaging 5 minutes each call, then that's $43.50 on Optus and $6.00 on MynetFone. So that's easily saved more than $30 even after taking away the $4.95 for the plan.
If you make calls overseas, with calls to most popular countries costing $0.10 for as long as you want - then the savings would be enormous. Voice quality and reliability have been equivalent (and sometimes better) than a normal landline.
Of course the amount you might save will depend on your circumstances - you might not have broadband, or just use a mobile with so much "included calls" that there's little incentive to use a landline, but definitely worth checking out otherwise and it costs little or even nothing to try. I'd also note that this makes for an incredibly easy and cheap way of setting up an extra telephone line for the kids or fax machine (no "I'll hang up now so you can fax me").
FYI we use an unlocked Linksys SP3102 (so people can call us on either the VoIP or standard number and it all goes through the the normal 'phone rather than using two different 'phones) and have setup a dial plan so interstate/international/mobile calls go via MynetFone and the rest go through the Optus landline. Probably more complicated than necessary, but then again I'm a geek and I recovered the additional cost of the hardware within 3 months... ;-)