IINM, Counterspell is an Instant card. The pecking order of spell effects is
Instant < Interrupt < Mana Source

So, an instant can be cast the same time as another Instant, but not for an Interrupt or Mana Source... So, yes a Counterspell can counter another Counterspell. It can't counter a Mana Sink or Dark Ritual.

There are no more interrupts and Mana Source? Have I been out of the loop that long? Seriously? Is this for the game or the actual CCG? If its the latter, that's a pretty big change in rules.

I last touched MtG around 2002 (Around the time the Affinity ability was introduced)... I think I still saw Interrupts... or maybe they were removed back then and I just never noticed.