I have one of the SEEK IR units for my android like Ben Witter mentioned above and for the sub 200 dollar price you can't beat it
but it will drain the phones battery quick if you use it a lot. I also have some of the FLIR units with much higher resolution but at a lot more cost... while they work great for just about anything you can think of like Camshawn says just for finding a hot bearing or certain electrical troubleshooting you can get away with less resolution and still make do... of course a regular IR gun with laser dot can also work it's just a matter of speed and convenience where a thermal picture is worth a thousand words of time saved
Have also used them for seeing how full tanks are, finding liquid flowing in pipes, helping people 'see' where they need more insulation in buildings and just about anything else where subtle temperature differences can tell a story and you don't have to be anywhere close to the area in question.
A lot of the newer offering of 'IR camera devices' you see out there lately all use the same FLIR 'Lepton' imager chip which is a fairly low resolution I think 60X80 or something like that... so to play with numbers to increase sales they do what I like to call 'cheating the specs' this is where they take a regular camera with higher resolution in the same device and mix the two pictures (regular and thermal) into one composite image and then show you a very good looking image showing the hot spots with plenty of detail!!!