JWPSF: The program
jwpsf models Point Spread Functions for the James Webb Space Telescope. It models monochromatic and broadband PSFs, optionally incorporating source spectra and filter functions. Source spectra, representing a range of stellar types, are included in the package plus a selection of idealized filter functions.
Astronomer's Proposal Tool (APT): APT is the operational tool for writing HST proposals and it is being modified to also write JWST proposals. You can download the current operational version of APT here and look at the current draft version for JWST by clicking on File>New>New JWST Proposal. Note that at this point the observation templates generally reflect what has been developed for the on board scripts and not necessarily all planned functionality.
JWST Prototype ETC: This JWST prototype Exposure Time Calculator (ETC) is a web-based application that assists users in exploring JWST's science capabilities. The ETC calculates Signal-To-Noise Ratio (SNR) for a given exposure time and for JWST's science instruments.