Get alerted whenever someone uses your Mac?
I found an article on Macworld that suggested two apps: 
Hidden 
Undercover

Their pricing is quite different. Undercover charges $49 for unlimited use on 1 device, while Hidden charges $2.50/month for 3 devices. If you want a free option, I think Mackeeper's Anti-Theft feature is free, but I don't trust them.

Update: I found a free app called Prey which looks much more legit than Mackeeper.