Being in the embedded world I have always tested products that go to OEM's or end clients, I have never tested a product that my company really use. Besides the fact that you can use and feel the product in the real world, what are the differences?
Yes. They are vastly different.
Testing code to be used only by a captive audience has a completely different set of risks, schedules, priorities, impacts, etc. "Good Enough" means something very different for an internal audience than it does for paying customers.
The impact of poor quality has a completely different dynamic.
Very often, the budget allocated is completely different.
I think the main difference is the availability of users who actually use your products. If you are shipping an internal system, you have access to users who actually used it day to day, and you can ask them questions, study their usage or get their direct feedback.
Working on an architectural piece, a library or an embedded component, you would only get that feedback from a "Man in the middle" and not the actual end users.
In addition to what Joe notes I have seen internal applications often get the "well it works here, and its only us so quality doesn't need to be stellar" attitude get used. This is wrong. Anything that goes out should be put under the same guidelines for quality, other people in the company are Customers to, and you can get to them easier and have a better opportunity to stop by and see how the tool/product is being used.
Often this is skipped or neglected, but I always think of it this way: if its not something you'd want on your computer at home, don't ship it.
I think that it is one and the same thing. If you are in a top software testing company, they will emphasize equally on both of the products. It is the quality that all matters. If we are able to get our in-house product tested well than only we can able to satisfy the customer needs.
All it depends on the management of the organization. If they are good enough to allocate time, budget and resources then definitely Internal projects will also be of the same quality.