Google's calendar charts are still in beta, but they provide already a nice heat map visualisation of calendar year data. The current development version of googleVis supports this new function via
gvisCalendar. Here is an example displaying daily stock price data.
For the code below to run you will require the developer version (≥ 0.5.0-4) of googleVis from GitHub and R ≥ 3.0.2.
I suppose the biggest current drawback is that the colours of the cells cannot be defined by the user. However, this should change with future versions of the Google Chart Tools. For more information and installation instructions see the googleVis project site and Google documentation.
Interestingly, the calendar chart looks very similar to the visualisation R. Wicklin and R. Allison from SAS used for the winning poster at the Data Expo 2009. Paul Bleicher created a function in R, based on lattice that creates a very similar output. You may recall David Smith's blog post about this.
R version 3.0.3 (2014-03-06) Platform: x86_64-apple-darwin10.8.0 (64-bit) locale:  en_GB.UTF-8/en_GB.UTF-8/en_GB.UTF-8/C/en_GB.UTF-8/en_GB.UTF-8 attached base packages:  grid stats graphics grDevices utils datasets methods  base other attached packages:  googleVis_0.5.0-4 chron_2.3-45 lattice_0.20-24 loaded via a namespace (and not attached):  RJSONIO_1.0-3 tools_3.0.3