Generally combined with stuff like undead the concept has not really gotten a good work out lately. There are a number of generic Tainted lands out there, for instance anywhere that evil dwells. The cave down the way is tainted by evil because of the goblins living there, slightly boring but the most easy for the players to fix seeing as they only have to kill off all the local goblins. Another common one is a place where the dead rise on their own. This is generally harder to fix seeing as unless there is some kind of object personifying the evil its just the land itself is cursed so badly you get zombies and skeletons unless you burn the dead. Those are both boring and over used though so lets see what else I can think of.
With some thinking I realized something better then undead. In the past things where thought to come from other stuff in a way that we now know is false but would be perfect for my purposes. An example of this would be that the Barnacle Goose was originally thought to develop from the Goose Barnacle. Quite an interesting theory to make up for the fact that they did not know about migration. Warping it to my purpose is as easy as changing the bugbear a little and whats this, someone already did the heavy lifting for me! Now I just make it so that instead of having to come from the special bugbear seed and needing to be carved and instead have it so that in tainted land there is a chance that a pumpkin will if left untended grow into one of these and you have a new type of tainted land. A place where pumpkins rise up and kill if left alone and all that needs to happen is one single plant with regular pumpkins out in the wild that seeds the area around it.
A good scenario is a witch in the forest that is causing problems for the town with a small group of bugbears. The Players can go and defeat the witch in her house and with her dying breath the witch can tell them they have only unleashed a worse fate upon the village. Players leave and the pumpkin patch that the witch had is left untended so suddenly within a year or two the town is swarmed with bugbears.
Another form of Tainted Land that I like has been done up a bit but that should not stop you. What I mean is the spell tainted land. Whether it was the site of a wizard war or the result of a magical experiment the land is now in constant flux as magical energies not meant for our ken flow through the land warping all that is and will be. A good example is Eberron with its Mournland. I particularly like the living spells from it as it really drives home the oddness of it. Also like the Mournland if you have a place like it don't give how they where actually formed. The moment you do the players will already have the rest of your campaign world cratered with them. If there is one thing I know about players its that if you give them the power to blow up the world one of them will have the need to try it out.
A good place for a limited tainted area that could be cleared by the players would be an old battle field where a massive loss of life occurred. Depending on how you do it this could either be some kind of undead area or you could go the more off the tracks root and yes I do mean "root". A land so saturated in blood that the very plants have been warped by its presences. This would be a good place to put all the different types of meat eating plants. Tainted Treants and druids would probably be there as well basking in the dark energies.
Don't go putting this type of Tainted land just anywhere. There should only be about one to three in your campaign world at any one time and if you want a really big one then a single area will do you well. The place they should grow is not just any battle field but a field where the dead and dying are so deep that the combatants could not help but walk on them. I am talking serious carnage here. If it is a large area you basically need it to be the battle site where a cross-planar war occurred to get a sufficient death toll.
To finish this topic off I will say that when using any kind of Tainted land you need to have control. Can you think of any good setting where there was more then one or so such places? No neither can I and its because if you have to many its no longer a good idea. Keep the numbers of them down to maybe two or three at best if they are small areas. One is good enough for a large place. Also as a general rule having whatever occurred to make it so unknown is a good idea and the farther in the past that it happened the more weight the place has. So one big place that occurred a time ago and no one knows how it happened exactly will allow you to get the most miles out of a Tainted land.