As a general rule mammals are neither poisonous nor venomous. (A poisonous organism is toxic when eaten and a venomous organism somehow applies a toxin to another living organism.) There are a few exceptions to this rule. There are two distinct groups of venomous mammals, the monotremes and shrews.

The monotremes are a group of mammals that consist of only two species, the duckbilled platypus and echidnas. Monotremes are the most archaic of all extant mammals. They have not developed nipples to facilitate suckling and they also lay eggs, from which their young hatch. The males of both species have spines on their hind legs which protrude from their ankles inward towards their body. They can use them by quickly bringing their legs together around the animal or part of the animal they are trying to poison. The females are completely non-venomous for both species.

The spines on the male echidnas are actually oriented in such a way that they can never use them. They are not very flexible or quick animals and can simply not make use of their venom. Echidnas are very similar to American porcupines and are well protected by spines. In addition to this they can bury themselves very quickly (often in under 30 seconds). They simply do not need to be venomous, and in fact, on any given male the spines may or may not be functional. The echidna’s spines are almost certainly an evolutionary leftover (you know like the human appendix) from a time when echidnas actually used their venom.
Danger to Humans: Echidnas are endangered and endemic to only Australia, so if you ever actually see one in the wild consider yourself very lucky. In addition to this if you really fucking try you just might1 be able to get yourself invenomated by one of these guys. That is after you dig it out of the ground and get punctured by dozens of needle sharp spines; hey did I forget to mention they use their deceptively strong claws to tear apart rotting logs. So if you actually do succeed in your insane quest to be poisoned by an echidna, then I congratulate you because you probably just did something that no human has ever done before.
(1 just might – you don’t have a chance in hell.)

The duckbilled platypus is another story. The males viciously use their spines on each other when fighting for territory or females. The actual effect that the venom has on male platypus isn’t known, but since they never seem to be permanently damaged due to the venom it can be assumed that they have strong natural resistance to it. Platypus always choose to run from predators, and not defend themselves with their spines. If a male platypus is left with no other choice it may sting its assailant, but this if far from their primary use.
Danger to Humans: The duckbilled platypus is extremely endangered and also endemic to only Australia. In addition to this they are very shy and wary. Even if you are in an area that they inhabit you will likely never see one. So let’s say you’re walking along some quiet, remote, unpolluted stream in Southern Australia and you see a platypus basking on the stream bank. My instructions are simple, (a) Thank God, Buddha, Darwin, or whoever because you are incredibly lucky; (b) Do NOT try to touch it or pick it up. This probably won’t be a problem since the critter will likely disappear into the water before you even get close. Only a handful of people have actually been stung by a platypus and how the venom affects the body is not well known. One Australian man (lets call him Angus), choose not to follow my advice. He saw a platypus lying on a log and was unsure of what it was, so he logically decided to pick it up. The fact that he was able to approach the animal probably means that it was sick, but not so sick that it couldn’t promptly turn and sting him as soon as he laid a finger on it. The venom of a platypus is very unique in that it directly stimulates nerve cells creating an extremely excruciating pain. In fact the term extremely excruciating is a grave understatement. It’s more like simultaneously being shot in the face and giving birth to quintuplets, except all that pain is centered around the place you were stung. So Angus, now in excruciating pain, went to the nearest hospital. Unfortunately he was in the wilderness and the nearest hospital was a small outfit with only one doctor on staff. The doctor had of course never treated someone for a platypus sting, but he did know how to deal with pain. The doctor gave Angus as much morphine as was safe. Though due to the unusual nature of platypus venom it had no affect on the pain, but I think Angus probably felt a little better with the morphine nonetheless. The doctor ended up having to stick a giant needle into the center of Angus’ elbow to paralyze the central nerve running to your hand (oh ya, Angus was stung in the hand). It was the only way to stop the pain. Angus never regained the full use of his hand because of extensive nerve damage done by the venom. (Based on a true story.)

There are also about five subspecies of shrews (possibly more) that are venomous. They not closely related to monotremes and use their venom in a completely different way. The venom in shrews is found in their saliva. They simply bite their prey and the some of their saliva/venom is injected into their victim. Shrews have extremely high metabolisms; they need to eat at least a third of their own bodyweight each day to survive. In addition to this they subsist only on predation, so being venomous is very beneficial for them. Shew venom is not very developed and is not their primary tool for subduing prey. They are simply very small, so it helps them tackle prey such ranging from large insects to small fish and amphibians.
Danger to Humans: Most shrews are not of the venomous variety, so if you see one don’t freak out. The venomous subspecies are also probably only endemic to North America and possibly Europe. The only way to get bitten is to try and handle one. They are very small and fast so its unlikely you will be able to catch one in the wild. Shrews are so small that they pose no danger even if you are bitten, although some reports say that bites from venomous shrews are painful.

Information gathered from Discovery Channel specials, magazines, and books I have read/viewed during my meager lifetime.