Depending on who you ask, the Greater Celandine is either an herb or a weed. In other words, it can be useful but it's also highly invasive so you don't really want too much of it in your garden. It will take over, and it can irritate sensitive skin.
The Greater Celandine has been used for centuries to handle a laundry list of ailments. People used to chew the roots for toothaches, make teas or tinctures from the leaves to use as a purgative, or even blend the juice with honey to somehow clean the eye and improve eyesight. PLEASE DO NOT ATTEMPT ANY OF THESE CURES! Just think of it this way: if it works as a purgative that means it's making you feel sick, in other words, it's toxic. Every part of the plant. Get the dose right and it might do what you want it to do, but it's practically impossible to determine what that correct dose should be. Since toxicity is determined by the dose, there are plenty of much safer and effective options available these days.
That said, there is one use of the Greater Celandine I believe is safe (no part of the plant is ingested, and the dosage is controlled if used as directed). The thick BRIGHT yellow sap can be applied topically on an area of skin which you want to attack: warts. Allegedly the sap is also useful for corns or ringworm, but I have never tried it for those conditions.
Step 1: Recognizing the Plant
In my Brooklyn garden these grow to about 18" though according to wikipedia they range from 12" to 47." Of course they start out much smaller, and to keep them under control I usually pull them out as soon as I spot them when they are 1 or 2 inches tall.
The leaves are lobbed, the small (approx 1/2" to 3/4") flowers are bright yellow with 4 petals and about 4-5 of the flowers will branch out from a single stem (the fancy term is "umbelliform cymes"). After they've been fertilized, the flowers turn into long skinny seed pods, much adored by ants. The stems are hairy, the roots are fairly shallow, but really you'll recognize this plant easily by snapping off a stem; if you see a BRIGHT yellow (or flaming orange) latex ooze out, you've hit the celandine jackpot.
The color of the sap can vary between yellow and orange, depending on the individual plant. It also tends to turn get darker the longer it is exposed to the air.
Step 2: Cure Your Wart
You need fresh herbs for this; I've tried cutting the Celandine in advance, but the sap flows best when it's been freshly picked.
If you are dealing with a plantar's wart, before you begin the treatment, soak your foot and then cut or pick off dead skin covering the wart.
Unlike the photos here, wear gloves, especially if you have sensitive skin.
Spread the sap on wart, twice daily for about 9 days. After you apply the latex, cover area with a bandaid.
If the surrounding skin becomes irritated, protect it with vaseline before you apply the latex (although this can prevent the bandaid from sticking, so it's best to be careful and apply the latex ONLY over the wart itself).
Be careful because the sap stains everything, and it doesn't keep its pretty color either (it turns dark brown or even black in a few hours.
This use of the Greater Celandine has been practiced for centuries, but my personal experimentation is somewhat limited. I took these pictures 2 years ago, and although this is my wart and foot, after a couple applications of the Greater Celandine I pretty much forgot about it because the wart didn't hurt. Over time -- but I'm not sure how long exactly -- it quietly went away. I think the celandine helped speed things up, but this experiment clearly does not qualify as scientific proof of the efficiency of the Greater Celandine -- which is a good part of the reason I shared it here. I'd love to hear how it works for you!