In cinder block basements I've used a sealant called UGL Drylok. It's a white, thick, paint that seals cinderblock walls and gives a nice white finish to the walls. My parent's house has it on their walls and they haven't had a leak in well over 20 years.
UGL drylok is easy to install but be VERY sure to have alot of ventilation. It works VERY well but has a VERY strong odor and you'll have to heavily ventilate the basement while doing it.
There are a few different ways I've seen this done.
The best way is to trench around the outside of the foundation down to the footer, seal the outside of the wall, and install a drainage system (gravel and pipes) at the footer to redirect the water to a sump pit where it can be pumped out.
Next best way is to do basicly the same thing, but on the inside of the house. You bust up the concrete floor of the basement along the walls and install gravel and PVC pipe under the floor to drain the water to a sump pit where it can be pumped out.
Both those options are going to be pretty expensive, and probably not something you would want to tackle on your own. Which is why many people opt to simply seal the inside of the walls with a product like Drylok as Scott mentions. It's pretty effective if you only have minor moisture problems, but if you regularly get lots of water in the basement, it's probably not going to solve your problem.
@eric-petroelje has it right: you need a weeping tile system on the outside of the foundation so that water drains away from the foundation at its base. Installing it after-the-fact means digging down to the footings around each wall in turn.
If you have a high water level, there is no product on the planet that can resist the hydro pressure from the water table. You need to divert the water, not resist it.
While the side of the foundation is exposed there are several products you can treat the exterior concrete with: essentially spray-on sealants that will give you a first-class job. I've only seen that on TV, though. The three times I've helped with this kind of job the exterior sealant was not applied and there was no leakage afterward.
There are companies that specialize in this: in my area installing weeping tile around the perimeter is a job in the 50k range. My dad did it to two of his rental houses using a spade, though: it can be done if you're not a lazy GenX-er like me.
If your moisture problems aren't severe, and you don't want to commit to an expensive job like this, you could see if a sump pump solves it for you. Essentially you dig a 1 ft square hole in the basement floor, down about four feet, and install a pump to discharge accumulated water outside before it becomes a problem. That's a solution somewhere between the DryLok approach and the weeping tile.