The OS gem allows for some easy telling if you're on windows or not.

    require 'os'
    
    >> OS.windows?
    => true   # or OS.doze?
    
    >> OS.bits
    => 32
    
    >> OS.java?
    => true # if you're running in jruby.  Also OS.jruby?
    
    >> OS.ruby_bin
    => "c:\ruby18\bin\ruby.exe" # or "/usr/local/bin/ruby" or what not
    
    >> OS.posix?
    => false
    
    >> OS.mac?
    => false
    
    >> OS.dev_null
    => "NUL" # or "/dev/null" depending on which platform
    
    >> OS.rss_bytes
    => 12300033 # number of rss bytes this process is using currently.  Basically "total in memory footprint" (doesn't include RAM used by the process that's in swap/page file)
    
    >> puts OS.report
    ==> # a yaml report of helpful values
    --- 
    arch: x86_64-darwin10.6.0
    target_os: darwin10.6.0
    target_vendor: apple
    target_cpu: x86_64
    target: x86_64-apple-darwin10.6.0
    host_os: darwin10.6.0
    host_vendor: apple
    host_cpu: i386
    host: i386-apple-darwin10.6.0
    RUBY_PLATFORM: x86_64-darwin10.6.0

   >> OS.cpu_count  
   => 2 # number of cores
  
If there are any other features you'd like, let me know, I'll do what I can to add them :)

http://github.com/rdp/os for feedback et al

Related projects:

rubygems:
  Gem::Platform.local
  Gem.ruby

the facets gem (has a class similar to rubygems, above)
  require 'facets/platform'
  Platform.local

the "platform" gem, itself (a different gem)

The reason Gem::Platform.local felt wrong to me is that it treated cygwin as windows--which for most build environments, is wrong.  Hence the creation of this.