So, I live on the second floor and the downstairs door is always locked. People want to drop by but it means they have to call my cell phone and, let's just say, I can get lazy about picking up the phone. Oops. So it seems that it might be time to look into a wireless doorbell. Turns out a) they make them, and b) they make some pretty killer ones.
Wireless Singing Bird Doorbell:
I loved this little bird when I saw it and then found out that he'll chirp a range of tunes including 'Mary Had a Little Lamb' and 'London Bridge is Falling Down' when someone is at the door. Install at your own risk! Buy it here.
Waleli GSM Doorbell (sort of wireless and by far the most complicated and it's pink!):
This doorbell calls your cell phone (wouldn't work so well for me...) allowing you to talk with the person at the door via an intercom. Then by punching the pin code into your phone, it unlocks the door. This means that not only can you let people in without having to get up, but you can let them in even if you aren't home (good for when you lock yourself out!). If you're out of town frequently and need to let the cat sitter in, this one's for you. More here.
Heath/Zenith Wireless Chime:
Just a straight up, no frills, wireless doorbell. Did I mention this one is cheap? Buy it here.
Portable Super Loud Doorbell with Two Receivers:
This one you can have chime in 2 different rooms, or you can carry one around on you (for when you're walking down the street maybe?). In case it wasn't obvious, this one is super loud and available here.