Weed Strains 3D: joinTogether This free interactive library of marijuana strains has information and tips for growing different strains. Good photos, very clear - but not very many strains in the library.
WeedLaws: Marijuana Law Guide This is a free pocket guide to Marijuana and Medical Marijuana laws in the US and PR. The accuracy is not very good - this shouldn't be used for reference.
Marijuana Encyclopedia This app costs US$1.99 and is a searchable database of over 320 strains of marijuana with descriptions of effects, taste, smell, genetics, THC content, etc. Could still use more strains.
Medical Marijuana GrowApp This free app has tips for growing medical grade marijuana at home. It's not very complete - but no one should expect to learn to grow marijuana from a free app. It's got a dictionary, a journal, and some other grow tools including troubleshooting.