OK, many people are asking about what's good and what's not.
The only way to find out for sure is to research yourself at impartial sites like Tom's Hardware Guide or Anandtech. But, to give you a head start, here is a list of manufacturers who have a good reputation for certain types of product:
AMD are good value for money but high-end Intel chips are slighlty faster (due to very high clock rate).
There are lots of threads in Hardwareworld about AMD/Intel - read them and do your own research before making a decision!
Asus are probably the best high-end motherboard manufacturer. For mid-range or budget systems, I've had good experiences with Elitegroup.
For Intel systems: the Granite bay chipset from Intel.
For AMD systems: nVidia nForce2 all the way!
Crucial have an excellent reputation for reliable RAM, as do Corsair (thanks for reminding me Soul184!
IDE Hard Drives
Western Digital and Maxtor make reliable, high, performance hard drives.
LiteOn and Teac are good.
nVidia Geforce cards are affordable and powerful, but ATi are currently the performance kings.
Two words: Creative Labs
Ilyama, Philips, and LG all have good reputations.
Logitech make good, reliable mice of all kinds.
Genius make very good budget mice - they make decent optical wheel mice for less than £10.
HP make the highest-quality printers but they can be expensive to buy and run.
Epson make less robust printers, but they are more inexpensive.
This is by no means an exhaustive list - I don't have enough experience to make recommendations about everything. Please add your own, but please be impartial and make sure your views are well-founded.