Obviously iPhoto is the 800 pound gorilla when it comes to browsing and archiving images on the Mac. iPhoto comes bundled with all Mac's and it's a decent enough application for most users. But it's the be-all-end-all of image browsing applications so let's look at some alternatives.
CocoViewX: This is a simplistic image browsing application. You open a directory, the files in that directory is listed on the left, and the bulk of the window is a preview of one of the files. It will do preview of images, PDF files and probably more. The operations you can do are very minimal (only rotation).
QPict: Is a simplistic image browsing application with a couple useful features. In the left of the window you have a tree view of the file system. If you click on a directory it scans the directory structure below it for all images, even if the images are in nested directories. The scanning is very quick even if there are hundreds of images involved. The application supports the RAW format from a large number of cameras, and will export RAW images to either TIFF or PNG. You can rotate images, add keywords to images, and you can search for images matching characteristics like keywords.
ImageBrowser: Is an almost bad piece of software. The window has a left section for browsing the images, and an image viewing area taking up most of the window. You open a directory and it lists the files in that directory. The only operation you can do is Make Thumbnails, not even image rotation. When you click on a file it's shown in the viewing area at a 1:1 zoom ratio, so if you have a large image as taken by any decent camera the picture is going to be larger than your monitor. Since the image is going to be larger than your monitor it would be most helpful if the image were scaled to fit the image area, rather than viewed in 1:1 mode. This means for every image you're going to click on the "zoom out" button so you can see the image. Because this application has such pitiful features and bad usability I heartily disreccommend it.
ImagePlus: Is a rather powerful image browsing and manipulation program. The layout is a little different, to the left is a tree structured view of your directories, at the bottom is a thumbnail listing of the images in the selected directory, and in the middle is the image previewarea. It automatically sizes images to the image preview area. It offers a good set of image manipulation features. Except that for the manipulation to be effected on the disk, apparently you must "save" the image. It even allows you to enter information that Spotlight will see.
Keyword Assistant for iPhoto: By all rights I shouldn't be listing this here but it looks like a useful program. It adds a capability to iPhoto so you can quickly add keywords to images. This feature should probably be built into iPhoto, but here we are.