From antiquity to modern days, the shield-wall has allowed a small number of trained fighters to control or defeat a horde of rabble. The concept is quite simple: The men stand in formation, shoulder to shoulder so that their shields overlap. This way an enemy is denied the ability to flank the fighters one by one. Throughout history this tactic has been used by amongst others the Persians, Spartans, Romans, Vikings, Saxons, Normans, and in our own age, riot police.
Lend an ear and an eye to this burly gentleman and his friends as they demonstrate the basics of the Saxon shield wall. The concept is pretty much the same for a Roman, a Viking, or a SWAT formation. The weapons may differ though.
This South Koreans riot police drill utilises the shield wall to advance, surround the opposition, neutralize the threat, and secure the area. I don't think the Romans could have done it better.
Most RPG systems have rules for shields, although I have yet to see something that fully utilizes this piece of equipment*. Most rules only offer the shield-bearer individual protection-bonuses, and while some have feats et al. that can include added protection for the side-man, these tend to be overly complicated or not efficient enough to justify the purchase. This really is too bad, as you don't need more than a handful of guys to pull off a proper wall.
Think dungeons. Three guys in front with overlapping shields can block the passage, and provide cover for magic users and spearmen in the back. Woe to the goblins who try to storm that party. In a modern setting, riot shields could be used to fend off vampires or slithering horrors from the stars while other investigators weave spells, fire handguns, or toss molotov-cocktails at the nasties.
*I seem to remember Pendragon having a good system for the shield wall, but as I haven't read that system in a decade I may be wrong here.
[Picture source: 1 unknown; 2 BBC News]