Linux Clustering Hardware?

Kanagawa asks: "The last few years have seen a slew of new Linux clustering and blade-server hardware solutions; they're being offered by the likes of HP, IBM, and smaller companies like Penguin Computing. We've been using the HP gear for awhile with mixed results and have decided to re-evaluate other solutions. We can't help but notice that the Google gear in our co-lo appears to be off-the-shelf motherboards screwed to aluminum shelves. So, it's making us curious. What have Slashdot's famed readers found to be reliable and cost effective for clustering? Do you prefer blade server forms, white-box rack mount units, or high-end multi-CPU servers? And, most importantly, what do you look for when making a choice?"