Here are some of my favourite restaurants in Helsinki. I try to visit as many of them as I can whenever I am in my old hometown.
Restaurant Juuri. Famous for its Finnish Tapas, "sapas". A very good introduction to local dishes, traditional ingredients but made with a modern innovative twist. Mid-priced, although I find the mains slightly overpriced and usually stick to "sapas"
www.juuri.fi (menu in English)
If the weather is nice, you should try one of the summer restaurants located in the islands, literally a couple of hundred yards from the city centre. They are pricey, but definitely something worth experiencing. Restaurant Saaristo (or Klippan, the name of the island) is one of the old favourites and is a 1 min boat ride from the centrally located Olympia terminal. The same company runs another place called Saari. I like both of them.
As another recent Chowhound post wrote, Restaurant Kuu is a nice local neighbourhood bistro. French-influenced dishes, but FInnish classics as well. Very good and very resonably priced Wine list. I think it has the cheapest champagne by the glass in Helsinki
For midpriced Finnish classic dishes such as fish (vendance and other delicious lakefish, Baltic herring and salmon) and meat, try any of the local no nonesense favourite restaurants:
Kosmos - http://www.ravintolakosmos.fi/englanti/englanti.htm
Elite - http://www.royalravintolat.com/elite/index_eng.asp
Sea Horse - www.seahorse.fi
For Michelin-star quality, the best restaurant in FInland is the excellent but pricey CHez Dominique (two stars).
For one star options I like Postres, athough its prices have gone up a bit too much in the recent years. I have heard good things about the new one star Luomo
www.luomo.fi - there is a sample menu in English under "Keittio".
Helsinki is an excellent place for Russian fare. The most famous, is the pricey Restaurant Saslik. My personal favourite is Restaurant Kasakka, which is a cheaper option. Both will fill you up with traditional russian fare.
I would recommend reservations to any of the places mentioned above. Have a nice time in Helsinki!