Well, Ruby/Saphire/Firered/Leafgreen/Emerald event Ho-oh can be obtained in Pokemon Colisseum and Heartgold (not sure if that counts) and R/S/FR/LG/E event Lugia can be obtained in Soulsilver and Pokemon XD, which seems to be about laughing hysterically.
Jirachi could be obtained from Pokemon channel too, without an event, as well as a Pokemon Colisseum bonus disc, i've heard. Celebi could apparently be found on a Japanese Pokemon Colisseum bonus disc, too.
In gen 4, Phione and Mew could be obtained by depositing certain amounts of mon into My Pokemon Ranch, while Darkrai was a bonus from Pokemon Ranger: Shadows of Almia. All without events. As far as I know, that's it.
Basically, no event Pokemon can be obtained in game later without hacking tickets and stuff. Though as I said, there are other ways to get some of the Mon without events.
That's all I got.
