Are there any (big) datasets for datamining in R considering that I want to build a sort of expert system application with practical usage? The domains may be: health data, lifestyle etc
R itself has a datasets package. Check out the R Datasets Package
Description: Base R datasets
Details: This package contains a variety of datasets. For a complete list, use library(help = "datasets").
Update: Thanks to @PatrickHoefler for pointing out the full list of easily imported R datasets.