In addition to the reviews you can find in the Apple App Store and Android Market, there are several excellent sources of information about apps.
Start by looking at this article from iPhoneLife Magazine that recommends a number of sites for findings Apps including MacWorld AppGuide and AppShopper.com.
You’ll find reviews of Apps for both iPhone and Android at Best Apps.
PCMag.com has an article with the 100 Best iPad Apps and another with the 50 Best Android Apps. Appolicious and AppBrain recommend Android apps. Huffington Post has published the Best-Rated Apps of 2011.
To keep up with news and reviews about tablets of all stripes, check out TabTimes.
Trending Apps section of Android Market (Screen shot by Larry Magid)