Xee is a pretty good replacement for the viewer functions of Irfanview. It will rotate images, and save them as different image file types, but it will not resize or crop.
However, amongst small fast Mac viewers it has the real virtue of being able to be started anywhere in a folder of images, and then move back and forth at will amongst the images. This is a feature that even the otherwise wonderful Preview doesn't have - instead you have to select the set of images you want to view first. Xee lets you do this on the fly if you wish.
Xee also has a virtue that iPhoto doesn't - you can set it to show all images at a default 100% zoom factor. So, you can set the default zoom to 100% and then can fly back and forth around a folder of images as you wish.
Honestly, if iPhoto could be set to display images at a default 100% zoom, it would be an excellent image viewer, but it can't and the result is I don't use it all!
For a basic and flexible image viewer (which is what I used Irfanview for most of the time when I was on a PC), with some limited editing capabilities, it is hard to beat Xee. See
http://www.macupdate.com/info.php/id/19978 to get a copy.