The Nest Protect smoke detector can do a lot of cool stuff: on top of normal smoke and CO2 detection, it also sends an alert to your phone when it detects a problem and can be configured to light your way if you pass under it in the dark. If its $119 MSRP has been deterring you, you're in luck: you can get one for $94 today on Rakuten.
This model is the wired version, meaning you'll need the proper wiring to install it. That said, it does include a battery backup to continue essential operations during a power failure. At $93.99, the Protect is a full $25 off its usual price.
To get the discount, you'll need a Rakuten account, but they don't cost anything, and you really ought to have one anyway; the site runs killer sales pretty frequently. Enter coupon code PRO5B at checkout to save.