The following foods are poisonous to guinea pigs:
- potato sprouts, potato peelings, and anything to do with a potato plant
- beans and anything to do with a bean plant
- anything to do with a tomato plant (the tomato itself is okay)
- anything to do with rhubarb
- dill (watch out for prepackaged "salad & herb" kits)
- flowers or any part of a flowering plant
- unidentified weeds
- any foods that were previously frozen
- fruit cores, pits, and seeds
For a lengthy list of plants that are poisonous to guinea pigs, check out Karen Bawoll's book, "A New Owner's Guide to Guinea Pigs." The ASPCA also has an all-species list of 17 common poisonous plants.
I do want to point out two items on Adrienne's list that are actually okay for guinea pigs in limited quantities and fed only occasionally: broccoli and (seedless) grapes.
As a final aside, if you or someone you know own dogs, cats, or horses, the ASPCA also has a database of toxic plants geared to those species. It's definitely another link to pass along.