"record entire execution of your Java app; play it back on any machine". Other features: time-travelling debugger -- step backwards, jump to any point in execution, designed for long running programs; post-execution logging -- add log statements after the program has run, and see what it would have logged. Looks extremely nifty, but I wonder how big those recording files get...

from Basho, via istvan. DTrace is becoming more compelling as a deep instrumentation/monitoring API -- I didn't realise disabled DTrace probes were virtually 0-overhead (a "2 NOOP instruction placeholder", apparently), that's nifty. Wonder if they've fixed the licensing mess, though

'Backdoor that gives you a scala shell over ssh on your jvm. The shell is not sandboxed, anyone access the shell can touch anything in the jvm and do anything the jvm can do including modifying and deleting files, etc.' nifty!

wraps strace(1) to summarise and aggregate I/O ops performed by a Linux process. looks pretty nifty (via Jeremy Zawodny)

'your online JSON Formatter'. useful. via JKeyes

injects via gdb. pretty cool, if it works; one comment notes that they couldn't use it on a Rails app