Stephanie,
I'm a big fan of shrimp, plecostimus, corydoras catfish, and snails. Different critters eat different types of algae, and different critters have different drawbacks. Here's a quick pro/con list of the 4 I mentioned:
Shrimp: Pros - they eat most types of algae, can survive on little/no food (they also enjoy microorganisms that naturally inhabit the tank), and have the lowest bio-load on a tank, save for snails. Cons - they are very delicate and suffer from abnormal water parameters more than other critters, plus they are the most docile critters in the tank so they hide, and often end up as snack food for more aggressive critters, like loaches.
Plecostimus: Pros - they decimate green spot, brown spot, and most other algae that grow on the tank glass. They're also very beautiful. Cons - they can get BIG (even a bristlenose pleco can get up to 4"-6" long, necessitating a 30G or bigger tank), they have spiny fins so grabbing them is a big no-no, they don't eat hair algae, brown beard algae, or blue-green algae, and when they get big they can uproot plants. NOTE: Plecos rasp on driftwood as part of their diet, and they need a hidey hole to chill in. If you don't have driftwood in your tank and you want a pleco, I suggest you get some.
Corydoras catfish: Pros: Great little guys, stay small, enjoy some (but not all) types of algae, and are very docile. Cons: They are a schooling fish so you need at least 6 for them to feel safe and comfy. Another option along the same lines is an otocynclus catfish (oto).
Snails: Pros - they are the lowest bio-load on the tank, until they die (then they flood the tank with obscene amounts of ammonia very quickly), they are rarely picked on by swimmers (though loaches have been known to eat snails), and they are pretty, especially Nerite snails. Cons - they take FOREVER to do ANYTHING, if you get the wrong type of snail they will breed out of control, and they are often pests/hitchhikers on plants that you buy from the pet store.
I personally have 3 of the above types - red cherry shrimps, otos, and pond snails as well as Malaysian Trumpet Snails (both considered pest snails). The Malaysians burrow into the substrate and clean the gravel for me, so I haven't vacuumed my tank in years.
Hope this helps!