Pair Programming
Pair programming evokes almost as much controversy as Macs vs PCs and Windows vs Linux ... some people love it, some hate it, most don't understand it or have never tried it.
Naresh Jain has a very well though out post on his blog explaining a lot of the aspects to consider.
Personally, I find that having someone sitting on my shoulder helps me to focus, gives me someone to immediately bounce ideas off, gives a second brain to interrupt me when I'm going wildly off the tracks, and generally keeps me from flights of fancy.
In fact I can't really see a disadvantage of pair programming, other than I do tend to avoid surfing and blog posting when I am being watched - which may be bad for me, but is much better for my client :)