Ditto what Jack said.
I always install sp_whoisactive and could not live without it (I run version 11.something). The most notable advantage over sp_who or sp_who2 is that you can see the active query that's running instead of the ambiguous "SELECT".
It is worth noting that they are similar but different animals. sp_whoisactive tells you about stuff running right now (which is why it's not named, sp_whoisSuspendedOrRunnableOrSleeping). My personal preference is to run them both like so:
P.S. sp_askbrent by Brent Ozar will be out soon. It's a fabulous troubleshooting tool. He's still developing it but I had a chance to play around with what he's done so far. Keep an eye out for that!
Edit: included a better pic 😉
"I cant stress enough the importance of switching from a sequential files mindset to set-based thinking. After you make the switch, you can spend your time tuning and optimizing your queries instead of maintaining lengthy, poor-performing code."
-- Itzik Ben-Gan 2001