yamb Well, at the moment here are some of the big ones:
85 Base Speed (max 295): home of Heracross which is often something that you would like to outspeed
100 Base Speed (max 328): Probably the most important, needed to outspeed Mew, Entei, Jirachi, Celebi, Victini, Shaymin, Tentacruel etc.
105 Base Speed and 108 Base Speed: home of Mienshao and Infernape respectively who are pretty big players
And lastly 120 Base Speed (max 372): Alakazam lives here as probably the fastest unboosted Pokemon in the tier (don't really worry about Mega-Aero as he is on a whole other level and you'll need a scarf to beat him)
Note that the all these speed tiers (other than Zam) also have a high likelihood of being Scarfed.