What can I do if someone else files a patent application for my invention?

You MAY have recourse if your contract did not carry an obligation of assignment of any inventions to the contractee. the AIA changes to patent law allow you to file if, within 1 yr of their application, you file an application for patent, and can show that the contractee "obtained the subject matter from the inventor". But you have only 1 year to file, and you have to be able to prove the contractee is a deriver (obtained the invention from you).
It may be more advantageous to seek a correction of inventorship to include you as an inventor and license (for a royalty) assignment of ownership to the contractee. The threat of applying for a patent as an original inventor and citing them as a deriver may incent them to negotiate joint ownership of any patent that issues on their application.

If you had an obligation to assign any inventions resulting from the work (they paid you for the work, after all, so you may be obligated to assign it to them) you are out of luck. They have complete right to apply for that patent, though they have an obligation to name you as an inventor or co-inventor or the inventorship is incorrect, and could subject them to future invalidation of their patent.