It probably depends on the country and distribution: mine had both Reader's Hub (linked to Zinio and Kobo) and Nexto.pl (linked to the most popular ebook-shop in my country), preinstalled and ready to use.
I installed Moon+ Reader and it seems good enough for some epub files I have lying around. There's also Kindle reader app that I use a lot, but this is because I own a Kindle, so most of my current reading is in Kindle's .azw (aka .mobi) format.
There's also Mantano reader, which I haven't tried, but judging from reviews it is one of the best. It is a paid app.
Isilo for Android also exists (this reader remembers the times of PalmOS, WinCE and other grandfather mobile OS's), but this would be of use only if you already have lots of books in the old PDB format.