EDIT: I'm an intern at a small local history museum. We have a budget of $0. Project overview:
I've got a bunch of old paper maps. When I say old, I mean 1800s and up. They've been scanned into TIFFs. I geo-referenced them and exported as GEOTIFFs. I've digitized the components of the maps we are focusing on (railroads, roads, etc).
The goal is to construct a web application (I've used ESRI's ArcGIS Online for a different web map project with CSV data) that allows a user to select layers (of years past) to view how transportation has developed in our county. Aside from a layer selector, layer opacity needs an option to be changed.
I've looked into MapBox API, but it lacks opacity sliders. QGIS Web Client looks promising and similar to the ArcGIS Server Client, but I wonder if it is too complicated for our end-users. I don't have much time to get this together, nor do I have much in the way of heavy programming experience.