magick is basically energy in nature. a spell is cast using this energy to create change in ones life. a simple rule of thumb is if it deals with energy it will probably work. a spell for luck, love, wealth, protection, healing and so on would work. [provided the spell is cast correctly and the items used work with the spell in question]
a spell to create physical change, such as becoming anything other than human, flying/levetation, teleportation, invisibility, and anything that isn't humanly possible will not work because magick needs to work within the limitations of nature.
then you've got some grey areas where some people feel it's possible, and others do not, such as weather and death spells. some feel since you're working with energies and natural forces, these spells are in the realms of possibility, where as others feel we as humans cannot and do not possess such power. [as this would make us somewhat god like being able to toy with a persons fate] for these, i would study and decide for yourself, but also heed the warning to think before you cast. say weather spells are real, and you choose to change the forecast from rain to sun in your area, moving whole weather patterns can result in a drought and/or flood in certain areas.
if you're going through the spell section on this site, i would avoid the Trick and Fantasy sections as these were set up to file away non-working spells. while there are one or two working ones in their [such as calling upon an astral beings energy for protection or luck] i would avoid it until you have a better understanding of magick.
in regards to your extraordinary comment, magick is rather powerful, and can do some amazing things, just don't expect any Harry Potter feats. [though personally i'm still trying to do that tea stiring trick from Practice Magic lol] while i can't think of any spell i've cast with awe inspiring results, i can think of a few little things involving candles and circle casting. when my one covenmate [who is very energetic and connected with fire] casts the circle, every single time the fire candle burns out withing 15 minutes while the others are perfectly fine. doesn't happen when anyone else casts the circle, just with her. it's a normal thing for me, but i remember one time a friend was observing our circle and he had a major freak out when he saw the fire candle [which was larger than the others] burn out. maybe you won't have the same reaction as he did, but it's a little something.