Bjango's iStat Menus is the gold standard of system monitoring tools. It's one of the first apps I bought when I switched back to the Mac, and it's one of the first apps I -- and almost every geeky/techy type I know -- installs when they get a new Mac. Originally designed as a Dashboard widget, iStat has since moved to the menu bar, and with iStat Menus 4, it's now completely, comfortably ensconced there with a bold new look and full on Retina support.
If you haven't used iStat Menus before, the gist is this -- it tells you everything you need to know, likely more than you need to know, about what's going on with your Mac. That includes CPU, memory, disk usage, network activity, battery status (if you're using a MacBook), and tons more. If you're already familiar with iStat Menus, new stuff includes per process network bandwidth monitoring, calendar events, history graphs across the board, more detailed GPU monitoring, the ability to change fan speed, SMART status monitoring to disks, and read/write IOPS to disks.
iStat Menus 4 offers a free 14-day trial. If you like it, you can buy it for $16. If you're an existing iStat Menus user, there's also upgrade pricing available.
Note: Bjango's Marc Edwards is a friend and co-host of mine, but I've been using and loving iStat since long before I met Marc. Still, don't take my word for it, grab the free trial and decide for yourself.