Outside of Species 8472 are there any other aliens that proved to be immune to Borg assimilation and/or successfully drove them off, never to return?
Probably not the answer that you were expecting, but Humans, and to a broader extent, all the Federation species, proved to be a hard challenge for the Borg Collective, repeatedly inflicting them defeats and set-backs.
Granted, from a biologic point of view they were able to assimilate Humans, but concerning your other requirement (successfully drive them off), the Federation proved to be really effective in fighting them.
If you consider their "mission statement", that goes like that:
We are the Borg. Lower your shields and surrender your ships. We will add your biological and technological distinctiveness to our own. Your culture will adapt to service us. Resistance is futile.
you can clearly understand that in no circumstances they will content themselves to assimilate just some individuals, their ultimate goal is to completely swallow entire cultures and civilizations, and they consider themselves so immensely powerful to be beyond every attempt to contrast them by basically everyone (resistance is futile, remember?); there are no conditions here, no possibilities to escape or to resist, the only absolute truth of their intentions is to completely defeat and subjugate a whole civilization and assimilate it into their Collective, to the point where no distinctiveness will be left: one absolute statement for one absolute goal.
And, if you look at the history of their interactions with the Federation, you can clearly see that they never managed to achieve this goal, and that in every major confrontation, in the end they were defeated and had to surrender their assimilation plans.
The most notable examples are:
The Battle of Wolf 359: despite a first and overwhelming victory in 2367, one the greatest defeats that the Federation ever suffered, the Borg were ultimately defeated before they could reach Earth and assimilate it.
The Battle of Sector 001: another attempt was made in 2373, and even this time the Borg Cube that assaulted Earth was defeated by Starfleet. A Borg Sphere managed to escape and travel back in time, leading to:
The attempted assimilation during First Contact: where the Sphere was destroyed by the Enterprise-E and the Borg were defeated in the subsequent attempt to gain control of the Starship itself, thus preventing them to change history and create a future where Earth would be fully assimilated by the 24th century.
Perhaps a bit cheap (not really "a" species), there have been several instances where assimilation of a Borg drone that has re-learned free will doesn't work as expected:
- Hugh - Although the Enterprise crew returned him to the Borg at the end of Season 5, we learned at the end of Season 6 that his sense of individuality acted like an infection to the rest of the Borg on his cube, and they all had to be cut loose.
- The Borg Cooperative - A spinter group of freed drones that, after reconnecting with each other to create the Cooperative, was able to overpower the Collective and self-destruct a Borg cube.
- Seven of Nine's sub-collective - Prior to Voyager, when stranded with a couple of other drones and cut off from the Collective, Seven and three other drones started to remember who they were. In order to maintain unity, Seven forcibly created what was supposed to be a temporary collective, but instead accidentally created a permanent sub-collected between those three drones. They were eventually able to break away from the main Borg Collective on their own.
- One - An accident combining Borg technology with the futuristic tech from the Doctor's holographic emitter, One was a Borg drone with technology far more advanced than the modern Collective. Because of the technology, and never having been part of the Collective, One chose to protect Voyager and was able to take control of a Borg sphere with little issue, destroying it and fatally injuring himself in the process.
While not a numerous species would Soong-type androids qualify?
They are officially recognized as lifeforms (see TNG:Measure of a Man), and have had multiple encounters with the Borg and we've seen at least one on-screen attempt for the Borg to assimilate a Soon-type android when they tried to assimilate Data in First Contact which can be classed as a spectacular failure, and I'm fairly sure that Data makes a comment about them being unable to break in to his Positronic net which is why they have to try and make him join willingly, suggesting that the classic forced-assimilation approach of the Collective isn't possible.
Ok, scraping the bottom of the barrel here.
In VOY: Think Tank, Costanza humble-brags about helping the people of Rivos V fight off the Borg (in return for the recipe for Zoth nut soup). We all know the Think Tank are not to be trusted, so, maybe Rivos V did OK, maybe not.
Another stretch could be the Brunali. When they engineered Icheb to destroy Borg, well... it worked. Voyager found Icheb on a destroyed cube. Granted the Brunali as a whole have already been assimilated by the Borg--and yes the original question asks about species that have successfully and I assume actively resisted the Borg--but consider that the Brunali have intentionally hidden their technology and adapted their lifestyle to hide from the Borg and engage in asymmetric war (suicide guerilla tactics). I understand that the OP is not looking for species that merely survived the Borg, such as the El-Aurians or Arturis' species. But at least the Brunali continue to preserve their culture and carry out attacks the best they can.
An even further stretch could be the Hirogen. In VOY: The Killing Game, Seven of Nine says to Standartenführer Karr:
One day the Borg will assimilate your species despite your arrogance. When that moment arrives, remember me.
Pretty sure she wasn't talking about Nazis. So, again, clearly some if not most of the Hirogen (Species 478) were already assimilated by the Borg, reducing them to a nomadic ship-borne species, yet the Hirogen continue to utilize a vast ancient communications network spanning the quadrant and Seven at least does not consider them fully "assimilated." Again, I admit they are not immune to the Borg and I don't know if they ever successfully drove them off, but, you know, life goes on.
I am in shock that no one mentioned Q. I am pretty sure they didn't get assimilated!