Is there any app available in the Android Market for Stack Exchange sites?
There are a couple of apps out there like Droidstack and StackWidget. Both of the apps can work together. I've just installed them, but so far you can't seem to answer or respond to questions with them. You can though, follow your questions and get notifications when you get answers. Droidstack is a question browser, and StackWidget lets you watch specific questions.
There is also TopStack for question browsing and SOflair for reputation monitoring. Having tried them.
The official Stack Exchange app is now available in the Play Store.
There just happens to be such a tool. It's not an actual application, but it works quite well with and is optimized for Android phones and tablets.
Some of the features you will find:
Disclaimer: I wrote it.
DroidStack seems to be the ticket. Android Market Link
There is a site specifically for apps that use the Stack Exchange API(s): Stack Apps.
If you're looking for Stack Exchange apps for Android, simply look under the android tag there.
I recently had the opportunity to try Stack Remote, which features an incredibly well-designed interface that looks really slick:
As is the case with all other apps that use the latest version of the API (2.1 at the time of this writing), there is no support for asking and answering questions. However, the app is handy for checking your inbox and quickly browsing questions.