I'm looking for a web app I can install to allow users to create simple forms, collect responses, and extract the data (e.g. as a CSV spreadsheet) in order to track travel, expenses, etc. Any recommendations for an existing web app, or approaches to implement it (e.g. with Rails)?
You don't mention a budget, so I'll give you my two favorites:
Wufoo - Paid (unless you have small requirements), but awesome. Non-install, but you create the forms there and then install them in your webpage. You can bulk-export to excel or CSV.
FormTools - Free, open-source, self-hosted. Does all that you need, but isn't as simple to use as Wufoo. However, it is simple to use and I like it a lot.
The only form-creating apps I know of are Wufoo, Pandaform, SurveyMonkey, FormSpring.me, and OpinionLab. But of these, only Wufoo will work for what you want (I think; you may want to check the other sites to see if they have advanced integration APIs like Wufoo does). However, there is also a 3rd-party Google Form API that can customize Google forms via webscraping the formkey and using the Google Data API.
Though unless these services offer additional features you need, I'd just code the forms yourself. It's not that hard (easier than integrating with another service at least), and even if you want flexible forms, there are ways to do that as well (and if you don't want to use EAV, you could try these options).