How to get from the Helsinki-Vantaa airport into Helsinki city centre quickly and without paying too much for it?
Bonus question: how about if the flight arrives in the wee hours (say, at 03:00 am)?
Daytime, there are two very good options:
Late at night & early in the morning (i.e., when 615 is not running), HSL bus 620 is your friend. It runs every half hour mostly, with last departure from airport at 03:55 and from city at 03:10 (Fri & Sat nights 03:40). But first departure is at 05:00 (from city 04:10), so essentially the whole night is covered, all week! Night fare (charged between 02:00-04:30) is 7€. The 620 departs from the exact same platforms as 615, and estimated travel time is 40-45 minutes. (The 620 also has departures during daytime; the main difference compared to 615 is slightly longer travel time.)
A taxi is always a convenient option, of course, but it's far from cheap (around 35-40€). There are some fixed rate / shared taxi services too that are slightly cheaper (e.g. Airport Taxi, "from 19.50€").
Edit: updated for 2015. Note that slightly different 615 and 620 timetables may apply in the middle of the summer. Check the Journey Planner to make sure, as it always has up-to-date info on the HSL services.
Rail connection starting 1 July 2015
The new Ring Rail Line opens this summer, bringing a rail connection between the airport and central Helsinki. It won’t really be faster than the current buses, but it’s probably easier, more straightforward for the occasional visitor.
As you can see in the schematic map below, you can take either I train or P train no matter which way you're going. To optimise travel time (save 5 minutes), choose I train from Helsinki and P train from airport. Both trains will run every 10 minutes in the daytime.
(NB: at least some bus of the existing bus lines will remain in operation and may be valid options even after the Ring Line opens. I’ll update this with more details later.)
Thank you for your interest in this question.
Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
Would you like to answer one of these unanswered questions instead?