Actually I've faced a similar problem myself - i.e. connecting multiple VT100 terminals to a single server, and USB dongles were nothing near to satisfactory solution. You need a lot of such dongles, they are rather expensive and there's a lot of problems with compatiblity. Plus you won't plug in more than you have USB ports anyway, so in most cases you're still stuck with 4 or so extra ports.

I know there are special I/O cards for PC that support/provide multiple serial ports. Linux supports quite a few, so let me list from the kernel...

Intelliport II/Plus
Comtrol RocketPort and RocketModem PCI boards
Cyclades-Z card
Digi International's Xx, Xeve, and Xem series of cards
Digiboard PC/Xe, PC/Xi, and PC/Xeve
Hayes ESP serial ports.
Moxa Intellio multiport serial card.
Moxa SmartIO multiport serial card.
Multi-Tech cards
SyncLink Multiport
SDL Communications RISCom/8 multiport card
Specialix IO8+ multiport card
Specialix SX and SI multiport serial cards
Specialix RIO, a smart serial card (128 ports)
Stallion cards

I know there are devices you can attach to one serial line to split it into more, but I have no idea where to look for these (except that you could build one yourself from multiport '51s) and how they are driven - they certainly require some protocol on top of RS232.