Best Linux Hardware Diagnostics?

An anonymous reader asks: "I've been running Linux for a little while and usually hardware problems have shown up quite easily - kernel panic, no module, no networking, etc. - but recently I've encountered some problems with network disk access causing very high load, which I think might be hardware related. Under Windows I'd fire up SANDRA or the like and run a full system scan. I did a quick search and nothing really stood out. I was wondering if any Linux gurus out there would like to share their expertise on Linux diagnostics?"