Of the free options, probably HKN.
Although Mojo shows a lot of promise (same author).
Not sure if Mojo is a ready version yet, or if it's still in development.

Quite a few like Octopus, but the developer has been gone for a while.
So there are no updates, and its getting hard to find a version of it.

If you need to invest in 5x Battlechests + 5x WotLK + 5x Cataclysm (assuming 5-boxing)...
And are paying 5x monthly subscriptions... that's not cheap, if you cannot really afford a pay option for the boxing software.
Overall, the hobby is pretty cheap compared to many others, but if $20 for a keyclone license or $36/year for an IS Boxer subscription is too much to spend, the Blizzard subscription is probably too much too.