The Stockholm metro is known as "the longest art gallery in the world," as most of its stations are decorated with some kind of artwork.
In my home town of Montreal, all metro stations are also decorated with art, but they come nowhere near close to the splendour of Stockholm's metro stations. If you happen to be in Stockholm on a rainy (or cold) day, you could do far worse than exploring the city's metro stations. It is like visiting a museum for the cost of a local transit ticket!
Start by downloading and printing this excellent PDF document describing the art in the Stockholm metro:
In summer, you can also take an interesting guided tour of the metro, but it will only take you to 4-5 stations due to obvious time constraints. For more information, check out my tip in "Off The Beaten Path" section.
P. S. I have by no means visited every one of Stockholm's metro stations, but my "top 3" are: 1) Kungsträdgården for its dark underground garden with classic sculptures; 2) Solna centrum, for its over-the-top colour scheme and many interesting objects; and 3) Thorildsplan for its colourful '80s video games-inspired mosaics.
Almost all subway stations have pieces of art as part of their decor and every now and then SL arranges guided tours on Thursdays and Sundays about these pieces. They're free if you have a valid SL ticket, otherwise it costs what the ticket costs. See schedules on the website at the "Website" field.
Edit Jul 2009: they have changed the schedules for the art tours. On Tuesdays, they show art along the Blue line, on Thursdays, along the Green line and on Saturdays, along the Red line.
An interesting piece of information regarding the third picture on this tip can be found here.
The 'blue line' on the underground
Because Swedish geology means bedrock, the underground is no exception. The blue underground line is deeper down than the rest and this means that it is not only physically cooler but gorgeous. The rock has been left all curvy since the blasting to create the underground track and this means you walk in rocky halls to get to the platforms. They are also painted in different colours and some have further decorations. Public transport meets art! There is a booklet about it which is for free in the ticket booths. Sorry for the dark pic - I'll work on it once I get to know the digital better.
The Stockholm Underground (Tunnelbana) is not only a way of transportation, it is also a place where you can see a lot of art. Sometimes it is a bit hidden and you have to keep your eyes open to find it, but when you start to look around you, above you, beside you, you will start to notice that it is everywhere! Here are some photos that I took at several stations: T-Centralen, Stadshagen, Fridhemsplan and Kungsträdgården. Nowadays about 130 artists are present through their art in the Tunnelbana. This can either be skulptures, mosaics or paintings. Maybe it is not all to your taste, but it certainly gives character to the place. And it made my trip with the Tunnelbana not only a trip from A to B, but also something worth to look at and discover.
Some of the stations that are known for their art are: Kungsträdgården, Östermalmstorg, Stadion, Rissne, Universitetet and Fridhemsplan.
As if to compensate for the absence of graffiti, many of Stockholm's T-Bana metro stations are adorned with contemporary art. Radhuset T-Bana station is particularly famous for its artworks.