AlchemyAPI (http://www.alchemyapi.com/) provides an online service for keyword extraction, but also for sentiment extraction, text categorization, etc. That means that all tools are available as web services. Read more.
CoBaLT is an application in which a corpus of texts can be loaded so as to be able to annotate its tokens (lemmatize and more). The annotation work in CoBaLT gives two products: an annotated corpus, and a lexicon consisting of word forms and their corresponding lemmata and such assigned to them. Read more.