Nice Groovy script for Hudson that checks which jobs are building or in the queue (got this from http://adrian.org.ar/ci/concurrency-in-hudson)

gistfile1.txt

//Jobs currently building or in the queue
jobs = hudson.model.Hudson.instance.items.findAll{job -> job.isBuilding()} + hudson.model.Hudson.instance.items.findAll{job -> job.isInQueue()}

//Keep only the names from the collected job objects
jobs = jobs.collect{x -> x.name}

//get current job name
currentJob = Thread.currentThread().executable.toString().split()[0]

//cut current job from the jobs list
jobs -= currentJob

//decide if there is any running (or waiting) job
res = true
if ((jobs.size())  > 0)
{
    res = false
    println "The following jobs are being executed or waiting for an executor"
    jobs.each{println it}
}

return res