I avoid my desk whenever possible during a class and like to be moving around or up at the board. This proves difficult when my computer and powerpoints are at the back of the room and I need to change between slides. Of course I can just buy a clicker (good ones fetch $50-70). However, I looked at the app store and found Rowmote Pro ($4.99), an app that not only works as a clicker but does pretty much everything your keyboard and mouse can do.
I’ve just finished a unit on spreadsheets and scatter plots and I was able to be at the board or walking around the room the whole time whilst working a functioning spreadsheet. It was awesome.
There aren’t many apps I can say this of (if any) but I now use Rowmote Pro in every lesson and it means I can be where I want to be at any point during the lesson.
What apps make your teaching life easier?
Pumphrey's Math 
Looks awesome – but it seems it only works with Macs? The computer in my classroom is a Windows machine – is there anything comparable that you know of?
Great question. Not aware of anything at present but have been pretty Mac centric for a while now.