Sorry. I meant to answer you all day. It really depends on what you need it for. For all intents and purposes (unless you want to spend boatloads of money), there are two main types: Dissection scopes and compound scopes.
Dissection scopes offer little magnification and are usually binocular. You get a lot more working distance, too, so you can put whole rocks under there if you want to. Depending on how much magnification you get, they are great for big stuff like copepod counts.
Compound scopes are the ones from high school biology. They offer little working distance but really high magnification. If you are trying to ID phyto, you need one of these (probably). Their magnification typically starts at 40x life size and goes up from there (that's a lot of magnification for a copepod, BTW).
I have an el cheapo Russian dissection scope. I hardly use it, though, because the focus for the camera is different than the focus for the eyepiece and it just drives me crazy. My camera can give me 5x life size, so I typically just use that instead of the dissection scope. I use a Celestron LCD as a compound scope most of the time:
http://www.amazon.com/Celestron-44340-Digital-Biological-Microscope/dp/B0014YNGCK The optics are only passable and the lighting is a joke (I supplement the lighting with a gooseneck halide microscope light), but the LCD makes my eyes happy and makes taking photos and videos easy. It has a really nice, solid stage, which is really pretty surprising.
I also have an old Bausch & Lomb compound that has lovely optics but needs a good cleaning and a hookup for my camera.