r <- as.data.frame(cellStats(x,mean))

library(raster) #load raster package

rasterlist <- list("path 1","path 2",...) #create list of raster file paths
outlist <- list() #create empty list to store outputs from loop

for (i in 1:length(rasterlist)) { #for each raster in rasterlist
   r <- raster(rasterlist[[i]]) #read element i of rasterlist into R
   val <- getValues(r) #get raster values
   m <- mean(val,na.rm=T) #remove NAs and compute mean
   outlist[[i]] <- c(rasterlist[[i]],m) #store raster path with mean
   return("complete")
}

df <- data.frame(do.call(rbind,outlist)) #convert list to data frame
colnames(df) <- c("raster path","mean")