How to check Jenkins build executor free memory?
Have a look at the Monitoring plugin. Here are some more details about it (from the linked page):


  
  Charts of memory, cpu, system load average, http response times by day, week, month, year or custom period
  Statistics of http requests with mean response times, mean cpu times, mean response size by request and by day, week, month, year or custom period
  Errors and logs
  Current http requests
  Threads
  Heap histogram (instances and sizes by class)
  Http sessions
  Process list of OS
  MBeans
  Actions for GC, heap dump and invalidate session(s)
  Report in html or pdf
  In English, German, French, Portuguese or Chinese
  Jenkins security
  For Jenkins nodes (slaves in general):
  
  
  The report for the nodes is available at http://yourhost/monitoring/nodes
  Charts aggregated for all nodes of memory, cpu, system load average, number of running builds, build queue length, build times by period
  Detailed statistics of the build times and of the build steps by period
  Threads, process list and MBeans for each nodes
  Heap histogram aggregated for all nodes
  
  For each individual node (each node in http://yourhost/computer), reports and actions are available from the "Monitoring" page in the contextual menu or in the detail of the node:
  
  
  Threads, process list, MBeans of that node only
  Heap histogram of that node
  Actions for GC, heap dump
  
  And more...
  


Credits: Assaf Lavie (who first mentioned it in a comment, but for some reason didn't post that as an actual answer ...).