If you spend time with R regularly then you should have the basics of data manipulation & plotting down within a couple of weeks. The more esoteric functions will take time to master. These are PDF cheat sheets that can be printed out. Keep them handy as you work with R.
help.start() - This first link doesn't actually take you to another site because it isn't a resource on the web, it's contained with R. To access it just type help.start() at the prompt and a collection of manuals will appear in your browser window. Pretty handy reference tool, eh?
R Reference Card - This is the most popular reference card. This same document is available from a number of sites and includes info on input & output, getting help, working with variables & arrays, plotting & more. You want this one.
R for Data Mining - This document was written by RDataMining.com. It includes info on classification, clustering, social network analysis, big data and more. This is very handy if you're interested in knowledge discovery & data mining.
R Style Guide - This resource is more than a cheat sheet. Google's internal R user community put together this guide for clean R code that covers syntax & conventions that are unique to R. I include it here because I've refered to it quite a bit in my own work. Your code will be easy to read & maintain if you follow these guidelines.