Here are some pics of the highly touted Age of Kings formations.  Personally, I think the best thing about them is that they're easy to use.

The Line Formation:  This is the default formation.   It puts your archers behind infantry and siege behind everything else. All units move at the speed of the slowest member, and in most cases it's as fast as your siege weapons.

I'll get some pics of different comibinations of units, with cavs, monks, spearmen, unique units, etc.

The Flank Formation:  I haven't used this one too much yet, so I don't know how effective it is.  Usually when I use flanking manuevers, my men are much to far apart to be in the same formation.  Looks cool though, doesn't it?

The Staggerd Formation:  A pretty useful formation for archers, so they aren't bunched all up for siege to wreak havoc on. 

The Box Formation:  Isn't this one neat?  The tough units, in this case the champions, line up around the outside of the box while the archers and skirmishers get inside on the next ring.  The Siege Rams take the middle so they're safe and secure from any pesky knights trying to quickly kill them.