Microsoft Research and Bing are jointly hosting the Speller Challenge. The goal is to build the best service that could propose alternative spellings for search queries submitted to Bing. Entries must be submitted for the challenge in the form of a REST-based web service, and they will be judged based on their expected F1 score against a test set sampled from real Bing queries.
For development purposes, they are making available a TREC evaluation dataset through their Web-NGram service. Refer to this page for detailed evaluation measures and REST service specs.
Time to start implementing!