Mine... for years now, has been IrfanView. I would recommend this program to anyone in a heartbeat! In fact I was even brave enough to install this program on my parents computer! And they can now use most of its simplest features... they're the type that still call me up because they need help checking email.
So what exactly is IrfanView?
"IrfanView is a very fast, small, compact and innovative FREEWARE (for non-commercial use) graphic viewer"
And how exactly do you pronounce IrfanView?
What is the IrfanView icon, is this a bear or devil or something else?
"This is a red CAT, a road cat. Note, I like cats, but this is a joke ;-) "
Basically, the simplest thing Irfanview allows you to do is view most image formats. Other basics include quickly cropping, resize, batch conversion, etc... I mean really, you just have to get this program, it does everything... I think they're working on a plug-in to make your morning coffee! (I wish)
It really is my favorite little program that I always keep around because it takes up pretty much no memory, so when you need to re-size a picture, and you don't feel like waiting for your memory hog graphics program to open, this comes in handy! Plus, this program is very user friendly... You really don't need to know much about graphics to do the basics... They also have a handy little forum, user created tutorials (in the FAQ section), and it comes in a ton of languages & skins!
Did I mention its FREE?