WG had 2 big hits in 2008, Tell Me and Nobody, and I suppose they got great popularity mostly in Asia, like Philippines (I think a famous prison's inmates did a group dance) and SE Asia, not in the west or USA.
In Korea they got popular as they appealed to a broader group age,
SNSD was doing well and selling more than WG, but also but was more limited to young people.
At that time there wasn't so many groups, there was BEG and Kara, who was rehashed during 2008 and only in 2009 their popularity took off in Korea, first with Rock U/Pretty Girl/Honey, then with Mister from 2nd album. F0r BEG, Abracadabra also brought them to international popularity (they were well known in Korea before).
Girl groups debuting in 2009 meant they were planned at least with 2 years in advance, it was great for them to appear with this boom and to which they helped, increasing variety: After School, 2NE1, T-ara, f(x), Secret, Rainbow, the still most relevant groups up to now (not sure about Rainbow...)
As for male groups, I don't know much, Big Bang, DBSK and SuJu were the leads, but at least in the west I think their popularity also took off during 2008/2009, at least in Europe.
In any case, 2009 was a great year for K-pop.