I'm just getting up to speed on R and thus far have just waded around, but here's my current list of interesting packages for financial applications.
I'd be very interested in other suggestions. It seems that some packages are not in CRAN but on R-Forge and therefore require a few extra steps to build into the package library
RODBC (CRAN) - good connector to ODBC databases
XTS / ZOO (CRAN) - absolutely necessary for working with financial time series
RQuantLib (CRAN) - haven't used yet, but a wrapper to some of the QuantLib functions, probably especially good for options pricing etc...
RTAQ (CRAN) - haven't used yet, but really promising for working with tick/quote data in R
PerformanceAnalytics (CRAN) - just starting to use. Seems helpful for calculating perfomance measures (sharpe etc...)
stockPortfolio (CRAN) - haven't used at all, looks interesting
IBrokers (CRAN) - connector to IB TWS APIs
portfolioSim (CRAN) - haven't used, seems similar to stockPortfolio
portfolio (CRAN) - required classes for portfolioSim
Blotter / QuantStrat / FinancialInstrument (R-Forge) - haven't used yet but they seem promising to simulate systems
IQFeed (GitHub -
https://github.com/bwlewis/iqfeed) - haven't been able to build the package yet, but very promising as a connector to an IQFeed data source