Any app that tells me to take regular breaks from working?

WorkRave is probably what you want

Workrave is a program that assists in the recovery and prevention of Repetitive Strain Injury (RSI). The program frequently alerts you to take micro-pauses, rest breaks and restricts you to your daily limit.

More screenshots

You could use notify-send + crontab.

Open the terminal and run
$ crontab -e

Then paste the code below
0 * * * * DISPLAY=:0.0 XAUTHORITY=~/.Xauthority notify-send "Go take a break."
save the file and exit.

That's all. You will receive the notification Go take a break every hour. You can change the message to whatever you want.

Looks like DrWright is what you need.

It's not available in the main repos, but it does have a PPA (info from omgubuntu.co.uk):

sudo add-apt-repository ppa:drwright/stable 
sudo apt-get update && sudo apt-get install drwright