Full disclosure: I work for Appcelerator.

There's an app Showcase at http://www.appcelerator.com/customers/app-showcase/, with apps submitted by our users. You could use this to get an idea of who's using it. Here are some I personally like:

http://goo.gl/MHhMKr http://goo.gl/xuMzyw http://goo.gl/vJC74J

Now, if you're looking to choose between the two, here's some information that could help you to better understand how to compare them.

Phonegap and Titanium are very different tools and approach the cross-platform problem in a very different way. Phonegap is a framework for building mobile web apps. You start with a clean slate, look for a UI framework like Sencha, KendoUI, etc., and start building your app, which is a small website with some added device features.

Titanium is very different, and more in the line of tools like Xojo (http://goo.gl/bQ86R3). Titanium's approach is that you are actually a native developer, and you think of your apps as if you were developing it using the platform-provided tools. That is, if you're building an iOS app, you'd probably look for the UINavigationController. This is exposed by Titanium as NavigationWindow. All the features of that container control are available to you, but instead of writing Objective C, you're writing JavaScript. This also means that you don't have to start looking for a UI framework like mentioned above. Your app will always look native because you NEVER emulate the UI or the screen transitions using CSS; it all comes straight from the SDK, and exposed to you as Titanium Javascript Objects. Some objects will translate across platforms like textboxes, buttons, etc, and some other like the CoverFlow and the ActionBar are exclusive to iOS and Android respectively.

Now the challenge (this is a positive challenge) for a developer is that since you're thinking -native-, you're actually building multiple apps from the same source code, just like Xojo mentioned above. This is the MAIN difference between building Titanium apps or web apps. With Titanium, you don't necessarily think about building an app that looks and behave exactly the same across platforms, because that's not what a native developer does (look at Facebook, Twitter, Evernote, etc. on iOS and Android). With Titanium you plan your app to use native containers, native UI patterns and native navigation paradigms, but the code that makes up the content of your app, and the modules and classes that make up the business logic of your app, are shared across all platforms, all written in Javascript.

So, if what you want to build is a web app, then Titanium is probably not your tool. If on the other hand you want to build native apps, just like those written in Objective C or Java, but cheat using Javascript, and build them at the same time, then Titanium is for you.