Yes, it is possible to enter St Petersburg without a visa if you are prepared to arrive by boat and book a very restrictive and very expensive group tour. Of course mostvisitors to Helsinki who want a day trip elsewhere now go to Tallinn instead as the Estonians abolished visa requirements for the British as soon as they threw out theSoviets. Ferries are cheap and quick and Tallinn Old Town can be covered in a day. Eventually the Russian visa system will collapse; St Petersburg will still be there when it does. It is of course worth the wait, although not the current visa hassle.
Neil Taylor

You can take a cruise from Helsinki to St Petersburg without a visa through a number of companies, e.g. Scantours. Cruises on the M.S. Kristina start on the evenings of 23 and 29 July and 4, 10, 16 and 19 August and give you two days in St Petersburg, with the ship as your hotel. However, you are not free to wander in the city and are only allowed to leave the ship on organised tours. You will certainly get a taste of this great city: a chance to see the palaces (e.g. Peterhof), the Hermitage, Peter and Paul Fortress and spend an evening at the ballet if you so desire.
Terry Coles