The Anomaly series might interest you. Though it's more of a tower assault rather than tower defense (but if I am not mistaken there is a game in the series where you play as the defending party).

iBomber series

Toy Soldiers games (especially the old ones: Toy Soldiers and Toy Soldiers: Cold War)

Defense Grid - The Awakening
is really good and DRM free in retail as well as at HumbleBundle.
I completely suck at it, but it's great nonetheless.

My all-time favourite will always be the first Plants vs. Zombies though.
I've got the DRM-free retail version and absolutely love it.

On gog I only know your two games and the Anomaly series.
And then there is http://www.gog.com/game/kingdom. It's not really tower defense but it has some TD-gameplay elements in it.

I know you didn't ask for it but there are a lot of TowerDefense-games for android platforms and if you or someone else want to try one out, give Radiant Defense a shot. For me it's one of the best TD-games, because you can define the paths on your own.
Sadly there are some special weapons you only get for money. I never bought/needed those although the game is sometimes really hard (but still doable).

The one's I've tried, I find irritating and/or boring. Like dumbed-down RTS games where you have much less tactical options.

Plants vs Zombies got something right though, as that is one tower defense game I really like. It is just fun to play, something that is for some reason missing in other tower defense games I've tried. If I could just understand why it feels so much fun to play, while something like Fieldrunners 1-2 or Anomaly: Warzone Earth doesn't.

The sequel, PvZ 2, was pretty bad though, as its game mechanism was geared to trying to make you buy powerups with microtransactions. The more you played it, the more it would depend on luck, hence you wouldn't beat the game with skill, but either by increasingly retrying over and over again, or buying the damn powerups in order to proceed.

That is one thing that seems similar in many (mobile) free-to-play games: much of the gameplay is based on luck, not skill. Hence you can't really just become so good in the game that you don't need to buy any powerups, extra lives or whatever. They just try to give you an illusion as if the gameplay is also about skill, like Candy Crush Saga.

Which is why I asked for non-mobile, non free-to-play-with-microtransaction games.

Thanks for the suggestions everyone. So, nothing on GOG?

There is this pixel junk monster on GoG

http://www.gog.com/game/pixeljunk_monsters_hd

The only one I ever liked besides Defender's Quest was this one .

Another Vote For Defense Grid! simply awesome

Space Run: Fast and Safe Delivery is a pretty good one. You've got a ship made of hexagon platforms and have to cram in shields, weapons, engines, and cargo in a manner where they all compliment each other. If you get stuck its possible to earn extra cash by redoing old missions (and earning a higher rating) to upgrade ship's systems.

I enjoyed Two Worlds II Castle defense. It does feel a little like a mobile game but is pretty solid gameplay wise.

http://knucklecracker.com/

Klumpen's already pointed out Defense Grid.

Orcs Must Die! and Dungeon Defenders are pretty good as well. Orcs Must Die! sadly is Steam-only, but Dungeon Defenders has been bundled on a few occasions and is DRM-free on Humble Store (albeit with an absolutely obscene price - €45, only available as a complete package with all DLC).

One I've also enjoyed is Fieldrunners. It's actually originally an iPad game and even on PC the game shows its mobile origins (I actually first played it on the PSP), but it's not F2P, there are no microtransactions, and the game actually does work better with a mouse than it does with a touchscreen. I've tried playing the game on Android and I hated it there.

I strongly suggest "Defense Grid" like others, which is one of the few "classic" Tower Defense I really liked (with "Defender's Quest"). Unfortunately, it's not on GOG...

I strongly suggest you to take a look at the Creeper World trilogy. I love the 3 games (finished the 3 games, and stil play them), but it is more a kind of RTS-TD (or "aggressive tower defense"). If you want a "light-looking TD", take the first : few kind of untis, simple graphics... It's doing the job perfectly. If you want more "advanced" version with more units, very nice musics and graphisms, take the 3. The 2 is interesting and very good too, but particular due to it's vertical orientation. But they're all VERY good.
There are demos if you're interested (and even more levels that you can play freely [url=http://www.gog.com/forum/general/particle_fleet_the_next_game_from_knuckle_cracker/post9]online).

Another big suggestion is <span class="bold">AI War</span>, available here on GOG. It's a very interesting and good game, not a TD a first glance (it's a RTS-4x in fact) but has a big part of tower defense. To summarize, it's a space 4x where you try to defeat an AI who detain almost all the galaxy, and one of your necessary action is to defend your systems like in tower defense games (you place buildings, defenses...) and the computer is regularly sending waves depending on the threat you're representing to it. If you like TD and 4X (and deep games), you should take a look.

Winged Sakura Mindy's Arc is very nice but can be hard (which is a good point too, but it's sometimes a little too hard when you go on hardest difficulties. But the normal difficulty is well balanced for what I played (I'm mid-game)). And it's not a "classic" tower defense too : it's a little like "Plant vs Zombies", but you're always active (I call it an "active tower defense") : the twist is that your units are summoned, so you have to manage how much time they'll stay on field, how much time before you can summon another of this kind, your available energy to summon, what is incoming...
I like it : it's smart, dynamic, it has nice musics, it's interesting to play... But it has some flaws/little issues (it can be a little grindy/unbalanced at moments for example).
There are some short cutsenes which may you think this is a visual novel game : don't worry, these are usually short and absolutely not the core of the game.
But it deserves a look if you like TD.

Finally, some thoughts....
Beware Planet Earth is not perfect (and not perfectly balanced), but fun enough to finish it at least one time. Not a priority, but if you can't find anything else, it can be worth to try for a low price.
Plant Vs Zombies is nice, but I didn't liked PC version at all (I loved the DS one) but the huge flaw of every versions of this game is that is is way too easy (and I can't really recommend it if you want any kind of challenge).

Edit : And I must add Sang-Froid to the list, which is good and that I had totally forgot :P